Divided-aperture infra-red spectral imaging system

ABSTRACT

Various embodiments disclosed herein describe a divided-aperture infrared spectral imaging (DAISI) system that is adapted to acquire multiple IR images of a scene with a single-shot (also referred to as a snapshot). The plurality of acquired images having different wavelength compositions that are obtained generally simultaneously. The system includes at least two optical channels that are spatially and spectrally different from one another. Each of the at least two optical channels are configured to transfer IR radiation incident on the optical system towards an optical FPA unit comprising at least two detector arrays disposed in the focal plane of two corresponding focusing lenses. The system further comprises at least one temperature reference source or surface that is used to dynamically calibrate the two detector arrays and compensate for a temperature difference between the two detector arrays.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/539,899, filed on Nov. 12, 2014, which claims benefit under 35 U.S.C.§ 119(e) of U.S. Provisional Application No. 61/903,075, filed on Nov.12, 2013 and titled “DIVIDED-APERTURE INFRA-RED SPECTRAL IMAGING SYSTEM”and which is also a continuation-in-part of International ApplicationNo. PCT/US/2013/041278, filed on May 16, 2013 and titled“DIVIDED-APERTURE INFRA-RED SPECTRAL IMAGING SYSTEM FOR CHEMICALDETECTION,” which claims benefit under 35 U.S.C. § 119(e) of U.S.Provisional Application No. 61/688,630, filed on May 18, 2012 and ofU.S. Provisional Application No. 61/764,776, filed on Feb. 14, 2013. Thedisclosure of each of the above identified application is incorporatedby reference herein in its entirety.

TECHNICAL FIELD

The present invention generally relates to a system and method for gascloud detection and, in particular, to a system and method of detectingspectral signatures of chemical compositions in a mid- and long-waveinfrared spectral region.

DESCRIPTION OF THE RELATED TECHNOLOGY

Spectral imaging systems and methods have applications in a variety offields. Spectral imaging systems and methods obtain a spectral image ofa scene in one or more regions of the electromagnetic spectrum to detectphenomena, identify material compositions or characterize processes. Thespectral image of the scene can be represented as a three-dimensionaldata cube where two axes of the cube represent two spatial dimensions ofthe scene and a third axes of the data cube represents spectralinformation of the scene in different wavelength regions. The data cubecan be processed using mathematical methods to obtain information aboutthe scene. Some of the existing spectral imaging systems generate thedata cube by scanning the scene in the spatial domain (e.g., by moving aslit across the horizontal dimensions of the scene) and/or spectraldomain (e.g., by scanning a wavelength dispersive element to obtainimages of the scene in different spectral regions). Such scanningapproaches acquire only a portion of the full data cube at a time. Theseportions of the full data cube are stored and then later processed togenerate a full data cube.

SUMMARY

The systems, methods and devices of this disclosure each have severalinnovative aspects, no single one of which is solely responsible for thedesirable attributes disclosed herein.

Various embodiments of the systems described herein provide an infrared(IR) imaging system for determining a concentration of a target chemicalspecies in an object (e.g., a gas plume). The imaging system includes(i) an optical system, having an optical focal plane array (FPA) unitconfigured to receive IR radiation from the object along at least two ormore optical channels defined by components of the optical system, theat least two or more optical channels being spatially and spectrallydifferent from one another; and (ii) a processor or processingelectronics configured to acquire multispectral optical datarepresenting said target chemical species from the received IR radiationin a single occurrence of data acquisition (or snapshot). The opticalFPA unit includes an array of photo-sensitive devices that are disposedat the focus of one or more lenses. In various embodiments, the array ofphoto-sensitive devices can include a two-dimensional imaging sensorthat is sensitive to radiation having wavelengths between 1 μm and 20 μm(for example, in mid infra-red wavelength range, long infra-redwavelength range, visible wavelength range, etc.). In variousembodiments, the array of photo-sensitive devices can include CCD orCMOS sensors, bolometers or other detectors that are sensitive toinfra-red radiation. The optical system may include an optical aperture(a boundary of which is defined to circumscribe or encompass the atleast two or more spatially distinct optical channels) and one or moreoptical filters. In various implementations, the one or more opticalfilters can comprise at least two spectrally-multiplexed filters. Eachof these optical filters can be associated with one of the at least twoor more optical channels and configured to transmit a portion of the IRradiation received in the associated optical channel. In variousembodiments, the one or more optical filters can be spectrallymultiplexed and may include, for example, at least one of a longpassoptical filter and a shortpass optical filter, or a band pass filter(with or without a combination with another filter such as a notchfilter, for example). The optical system may further include at leasttwo reimaging lenses. The at least two reimaging lenses, for exampleeach of the reimaging lens, may be disposed to transmit IR radiation(for example, between about 1 micron and about 20 microns), that hasbeen transmitted through a corresponding optical filter towards theoptical FPA unit. In one embodiment, the optical FPA unit is positionedto receive IR radiation from the object through the at least tworeimaging lenses to form respectively-corresponding two or more sets ofimaging data representing the object. The processor or processingelectronics is configured to acquire this optical data from the two ormore sets of imaging data. In various embodiments of the imagingsystems, the FPA unit may be devoid of cooling systems. In variousembodiments, two or more of the array of photo-sensitive devices may beuncooled. In some embodiments, the system further comprises two or moretemperature-controlled shutters removably positioned to block IRradiation incident onto the optical system from the object.

Also disclosed herein is an implementation of a method of operating aninfrared (IR) imaging system. The method includes receiving IR radiationfrom an object through at least two optical channels defined bycomponents of an optical system of the IR imaging system, which at leasttwo optical channels are spatially and spectrally different from oneanother. The method further includes transmitting the received IRradiation towards an optical focal plane array (FPA) unit that is notbeing cooled in the course of normal operation. For example, in variousembodiments of the imaging systems, the FPA unit may be devoid ofcooling systems. In various embodiments, two or more of the array ofphoto-sensitive devices may be uncooled. Some embodiments furthercomprise removably positioning at least one of at least twotemperature-controlled shutters in front of the optical system to blockIR radiation incident onto the optical system from the object.

Various innovative aspects of the subject matter described in thisdisclosure can be implemented in the following embodiments:

Embodiment 1

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of cameras;    -   at least one thermal reference source having a known temperature        placed in front of the plurality of cameras and configured to be        imaged by the plurality of cameras; and    -   a data-processing unit comprising a processor, the imaging        system configured to:    -   acquire with the plurality of cameras a plurality of frames        having regions that correspond to the image of the reference        source; and    -   apply a dynamic calibration correction to the plurality of        cameras to allow every camera in the plurality of cameras to be        calibrated to agree with every other camera in the plurality        imaging the reference source.

Embodiment 2

The system of Embodiment 1, wherein the plurality of cameras comprises aFPA unit and a plurality of lenses.

Embodiment 3

The system of any of Embodiments 1-2, wherein the FPA unit comprises oneFPA or a plurality of FPAs.

Embodiment 4

The system of any of Embodiments 1-3, wherein the at least one thermalreference source has a known spectrum.

Embodiment 5

The system of any of Embodiments 1-4, further comprising an additionalthermal reference source imaged by the plurality of cameras.

Embodiment 6

The system of any of Embodiments 1-5, wherein the additional referencesource has a temperature and a spectrum different from the knowntemperature and the known spectrum of the at least one reference source.

Embodiment 7

The system of any of Embodiments 1-6, wherein the temperature of theadditional thermal reference source is less than the known temperature.

Embodiment 8

The system of any of Embodiments 1-7, wherein the temperature of theadditional thermal reference source is greater than the knowntemperature.

Embodiment 9

The system of any of Embodiments 1-8, wherein the at least one referencesource is displaced away from a conjugate image plane of the pluralityof cameras such that the image of the at least one reference sourcecaptured by the plurality of cameras is blurred.

Embodiment 10

The system of any of Embodiments 1-9, wherein the at least one referencesource is positioned at a conjugate image plane of the plurality ofcameras.

Embodiment 11

The system of any of Embodiments 1-10, further comprising a plurality ofmirrors configured to image the at least one reference source onto theplurality of cameras.

Embodiment 12

The system of any of Embodiments 1-11, wherein the plurality of mirrorsare disposed outside a central field of view of the plurality ofcameras.

Embodiment 13

The system of any of Embodiments 1-12, further comprising a first and asecond temperature-controlled shutter removably positioned to block IRradiation incident on the system from reaching the plurality of cameras.

Embodiment 14

The system of any of Embodiments 1-13, wherein the system includes atleast two spatially and spectrally different optical channels.

Embodiment 15

The system of any of Embodiments 1-14, wherein the system includes atleast three optical channels.

Embodiment 16

The system of any of Embodiments 1-15, wherein the system includes atleast four optical channels.

Embodiment 17

The system of any of Embodiments 1-16, wherein the system includes atleast five optical channels.

Embodiment 18

The system of any of Embodiments 1-17, wherein the system includes atleast six optical channels.

Embodiment 19

The system of any of Embodiments 1-18, wherein the system includes atleast seven optical channels.

Embodiment 20

The system of any of Embodiments 1-19, wherein the system includes atleast eight optical channels.

Embodiment 21

The system of any of Embodiments 1-20, wherein the system includes atleast nine optical channels.

Embodiment 22

The system of any of Embodiments 1-21, wherein the system includes atleast ten optical channels.

Embodiment 23

The system of any of Embodiments 1-22, wherein the system includes atleast twelve optical channels.

Embodiment 24

The system of any of Embodiments 1-23, further comprising one or moresensors configured to measure a temperature of the at least onereference source.

Embodiment 25

The system of any of Embodiments 1-24, wherein the plurality of camerasis configured to image the same portion of the at least one referencesource.

Embodiment 26

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of cameras;    -   a first temperature-controlled reference source imaged by the        plurality of cameras;    -   a second temperature-controlled reference source imaged by the        plurality of cameras; and    -   a data-processing unit comprising a processor, said        data-processing unit configured to:    -   acquire with the plurality of cameras a plurality of frames        having regions that correspond to the image of the reference        source; and    -   dynamically calibrate the plurality of cameras so that various        cameras imaging a scene are forced to agree on a temperature        estimate of the first and second reference sources.

Embodiment 27

The imaging system of any of Embodiment 26, wherein the data-processingunit is configured to calculate a dynamic calibration correction andapply the correction to the plurality of cameras for each of theplurality of frames.

Embodiment 28

The system of any of Embodiments 26-27, wherein the first referencesource is maintained at a first temperature.

Embodiment 29

The system of any of Embodiments 26-28, wherein the second referencesource is maintained at a second temperature.

Embodiment 30

The system of any of Embodiments 26-29, wherein the first temperature isgreater than the second temperature.

Embodiment 31

The system of any of Embodiments 26-30, wherein the first temperature isless than the second temperature.

Embodiment 32

The system of any of Embodiments 26-31, wherein the first and the secondreference sources are displaced away from a conjugate image plane of theplurality of cameras such that the image of the first and the secondreference sources captured by the plurality of cameras is blurred.

Embodiment 33

The system of any of Embodiments 26-32, wherein the first and the secondreference sources are positioned at a conjugate image plane of theplurality of cameras.

Embodiment 34

The system of any of Embodiments 26-33, further comprising:

-   -   a first mirror configured to image the first reference onto the        plurality of cameras; and    -   a second mirror configured to image the second reference source        onto the plurality of cameras.

Embodiment 35

The system of any of Embodiments 26-34, further comprising a first and asecond temperature-controlled shutter removably positioned to block IRradiation incident on the system from reaching the plurality of cameras.

Embodiment 36

The system of any of Embodiments 26-35, wherein the system includes atleast two spatially and spectrally different optical channels.

Embodiment 37

The system of any of Embodiments 26-36, wherein the system includes atleast four optical channels.

Embodiment 38

The system of any of Embodiments 26-37, wherein the system includes atleast six optical channels.

Embodiment 39

The system of any of Embodiments 26-38, wherein the system includes atleast eight optical channels.

Embodiment 40

The system of any of Embodiments 26-39, wherein the system includes atleast ten optical channels.

Embodiment 41

The system of any of Embodiments 26-40, wherein the system includes atleast twelve optical channels.

Embodiment 42

The system of any of Embodiments 26-41, further comprising one or moresensors configured to measure a temperature of the first or the secondreference source.

Embodiment 43

The system of any of Embodiments 26-42, wherein the plurality of camerasis configured to image the same portion of the first reference sourceand wherein plurality of cameras is configured to image the same portionof the second reference source.

Embodiment 44

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of cameras;    -   a reference having an unknown temperature configured to be        imaged by the plurality of cameras; and    -   a data-processing unit comprising a processor, the imaging        system configured to:    -   acquire with the plurality of cameras a plurality of frames        having regions that correspond to the image of the reference;    -   calculate a dynamic calibration correction using a temperature        measured by one of the cameras in the plurality of cameras as a        reference temperature; and    -   apply the calibration correction to the other cameras in the        plurality of cameras to match the temperature estimate of the        other cameras in the plurality of cameras with the reference        temperature.

Embodiment 45

The system of any of Embodiment 44, wherein the reference is displacedaway from a conjugate image plane of the plurality of cameras such thatthe image of the reference source captured by the plurality of camerasis blurred.

Embodiment 46

The system of any of Embodiments 44-45, wherein the reference ispositioned at a conjugate image plane of the plurality of cameras.

Embodiment 47

The system of any of Embodiments 44-46, further comprising a pluralityof mirrors configured to image the reference onto the plurality ofcameras.

Embodiment 48

The system of any of Embodiments 44-47, further comprising a first and asecond temperature-controlled shutter removably positioned to block IRradiation incident on the system from reaching the plurality of cameras.

Embodiment 49

The system of any of Embodiments 44-48, wherein the system includes atleast two spatially and spectrally different optical channels.

Embodiment 50

The system of any of Embodiments 44-49, wherein the system includes atleast three optical channels.

Embodiment 51

The system of any of Embodiments 44-50, wherein the system includes atleast four optical channels.

Embodiment 52

The system of any of Embodiments 44-51, wherein the system includes atleast five optical channels.

Embodiment 53

The system of any of Embodiments 44-52, wherein the system includes atleast six optical channels.

Embodiment 54

The system of any of Embodiments 44-52, wherein the system includes atleast seven optical channels.

Embodiment 55

The system of any of Embodiments 44-53, wherein the system includes atleast eight optical channels.

Embodiment 56

The system of any of Embodiments 44-54, wherein the system includes atleast nine optical channels.

Embodiment 57

The system of any of Embodiments 44-55, wherein the system includes atleast ten optical channels.

Embodiment 58

The system of any of Embodiments 44-56, wherein the system includes atleast twelve optical channels.

Embodiment 59

The system of any of Embodiments 44-57, wherein the plurality of camerasis configured to image the same portion of the reference.

Embodiment 60

An infrared (IR) imaging system, the imaging system comprising:

-   -   an optical system including an optical focal plane array (FPA)        unit, the optical system includes components associated with at        least two optical channels, said at least two optical channels        being spatially and spectrally different from one another, each        of the at least two optical channels positioned to transfer IR        radiation incident on the optical system towards the optical FPA        unit, the optical FPA unit comprising at least two detector        arrays disposed at a distance from two corresponding focusing        lenses;    -   at least one thermal reference having a known temperature,        wherein radiation emitted from the at least one reference is        directed towards the optical FPA unit and imaged by the at least        two detector arrays; and    -   a data-processing unit, said data-processing unit configured to:    -   acquire a plurality of frames with the at least two detector        arrays having regions in the plurality of image frames that        correspond to the image of the reference; and    -   dynamically calibrate the at least two detector arrays to        address a difference in the temperature estimate of the        reference between the two detector arrays.

Embodiment 61

The system of any of Embodiment 60, wherein the at least one thermalreference has a known spectrum.

Embodiment 62

The system of any of Embodiments 60-61, further comprising an additionalthermal reference, wherein radiation from the additional reference isdirected towards the optical FPA unit and imaged by the at least twodetector arrays.

Embodiment 63

The system of any of Embodiments 60-62, wherein the additional referencehas a temperature and a spectrum different from the known temperatureand the known spectrum of the at least one reference source.

Embodiment 64

The system of any of Embodiments 60-63, wherein the temperature of theadditional thermal reference is less than the known temperature.

Embodiment 65

The system of any of Embodiments 60-64, wherein the temperature of theadditional thermal reference is greater than the known temperature.

Embodiment 66

The system of any of Embodiments 60-65, wherein the at least onereference is displaced away from a conjugate image plane of the at leasttwo detector arrays such that the image of the at least one referencecaptured by the at least two detector arrays is defocused.

Embodiment 67

The system of any of Embodiments 60-66, wherein the at least onereference is positioned at a conjugate image plane of the at least twodetector arrays such that the image of the at least one referencecaptured by the at least two detector arrays is focused.

Embodiment 68

The system of any of Embodiments 60-67, further comprising at least tworeflecting elements configured to direct radiation from the at least onereference source toward the at least two detector arrays.

Embodiment 69

The system of any of Embodiments 60-68, wherein the at least tworeflecting elements are disposed outside a central field of view of theat least two detector arrays.

Embodiment 70

The system of any of Embodiments 60-69, further comprising a thirddetector array disposed between the at least two detector arrays.

Embodiment 71

The system of any of Embodiments 60-70, wherein the data-processing unitis configured to:

-   -   acquire a plurality of frames using the third detector array;        and    -   dynamically calibrate the third detector array to match a        temperature estimate of the third detector array with the        temperature estimates of the at least two detector arrays.

Embodiment 72

The system of any of Embodiments 60-71, wherein radiation emitted fromthe at least one reference source is not imaged by the third detectorarray.

Embodiment 73

The system of any of Embodiments 60-72, wherein the third detector arrayhas a field of view greater than a field of view of the at least twodetector arrays.

Embodiment 74

The system of any of Embodiments 60-73, wherein the at least onereference is imaged by the third detector array.

Embodiment 75

The system of any of Embodiments 60-74, further comprising a thirdreflecting element disposed outside the field of view of the at leasttwo detector arrays and configured to image the at least one referenceonto the third detector array.

Embodiment 76

The system of any of Embodiments 60-75, further comprising a first and asecond temperature-controlled element removably positioned to block IRradiation incident on the optical system from reaching the optical FPAunit.

Embodiment 77

The system of any of Embodiments 60-76, wherein the optical systemincludes components associated with three optical channels.

Embodiment 78

The system of any of Embodiments 60-77 wherein the optical systemincludes components associated with four optical channels.

Embodiment 79

The system of any of Embodiments 60-78, wherein the optical systemincludes components associated with six optical channels.

Embodiment 80

The system of any of Embodiments 60-79, wherein the optical systemincludes components associated with eight optical channels.

Embodiment 81

The system of any of Embodiments 60-80, wherein the optical systemincludes components associated with ten optical channels.

Embodiment 82

The system of any of Embodiments 60-81, wherein the optical systemincludes components associated with twelve optical channels.

Embodiment 83

The system of any of Embodiments 60-82, wherein the optical systemincludes components associated with sixteen optical channels.

Embodiment 84

The system of any of Embodiments 60-83, wherein the optical systemincludes components associated with twenty four optical channels.

Embodiment 85

The system of any of Embodiments 60-84, wherein each of the at least twodetector arrays is configured to image the same portion of the at leastone reference so as to consistently provide a common referencetemperature.

Embodiment 86

The system of any of Embodiments 60-85, wherein the data-processing unitcomprises processing electronics.

Embodiment 87

The system of any of Embodiments 60-86, wherein the data-processing unitcomprises a processor.

Embodiment 88

The system of any of Embodiments 60-87, wherein the data-processing unitcomprises one or more processors.

Embodiment 89

An infrared (IR) imaging system, the imaging system comprising:

-   -   an optical system including an optical focal plane array (FPA)        unit, the optical system including at least two optical        channels, said at least two optical channels being spatially and        spectrally different from one another, each of the at least two        optical channels positioned to transfer IR radiation incident on        the optical system towards the optical FPA unit, the optical FPA        unit comprising at least two detector arrays disposed at a        distance from two corresponding focusing lenses;    -   a first temperature-controlled reference imaged by the at least        two detector arrays;    -   a second temperature-controlled reference imaged by the at least        two detector arrays; and    -   a data-processing unit configured to:    -   acquire a plurality of frames with the at least two detector        arrays having regions in the plurality of image frames that        correspond to the image of the first and second references; and    -   dynamically calibrate the at least two detector arrays to        address a difference in a temperature estimate of the first and        second references between the two detector arrays.

Embodiment 90

The system of Embodiment 89, wherein the first reference is maintainedat a first temperature.

Embodiment 91

The system of any of Embodiments 89-90, wherein the second reference ismaintained at a second temperature.

Embodiment 92

The system of any of Embodiments 89-91, wherein the first temperature isgreater than the second temperature.

Embodiment 93

The system of any of Embodiments 89-92, wherein the first temperature isless than the second temperature.

Embodiment 94

The system of any of Embodiments 89-93, wherein the first and the secondreferences are displaced away from a conjugate image plane of the atleast two detector arrays such that the image of the first and thesecond references captured by the at least two detector arrays isdefocused.

Embodiment 95

The system of any of Embodiments 89-94, wherein the first and the secondreferences are positioned at a conjugate image plane of the at least twodetector arrays such that the image of the first and the secondreferences captured by the at least two detector arrays is focused.

Embodiment 96

The system of any of Embodiments 89-95, further comprising:

-   -   a first reflecting element configured to direct radiation from        the first reference toward the at least two detector arrays; and    -   a second reflecting element configured to direct radiation from        the second reference toward the at least two detector arrays.

Embodiment 97

The system of any of Embodiments 89-96, wherein the first reflectingelement is disposed outside a field of view of the at least two detectorarrays.

Embodiment 98

The system of any of Embodiments 89-97, wherein the second reflectingelement is disposed outside a field of view of the at least two detectorarrays.

Embodiment 99

The system of any of Embodiments 89-98, further comprising a thirddetector array disposed between the at least two detector arrays.

Embodiment 100

The system of any of Embodiments 89-99, wherein the data-processing unitis configured to:

-   -   acquire a plurality of frames using the third detector array;        and    -   dynamically calibrate the third detector array to address a        difference in the temperature estimate of the first and second        references between the third detector array and the two detector        arrays.

Embodiment 101

The system of any of Embodiments 89-100, wherein the first and secondreferences are not imaged by the third detector array.

Embodiment 102

The system of any of Embodiments 89-101, wherein the third detectorarray has a field of view greater than a field of view of the at leasttwo detector arrays.

Embodiment 103

The system of any of Embodiments 89-102, wherein r first and secondreferences are imaged by the third detector array.

Embodiment 104

The system of any of Embodiments 89-103, further comprising a thirdreflecting element disposed outside the field of view of the at leasttwo detector arrays and configured to image the first and secondreferences onto the third detector array.

Embodiment 105

The system of any of Embodiments 89-104, further comprising a first anda second temperature-controlled element removably positioned to block IRradiation incident on the optical system from reaching the optical FPAunit.

Embodiment 106

The system of any of Embodiments 89-105, wherein the optical systemincludes components associated with three optical channels.

Embodiment 107

The system of any of Embodiments 89-106, wherein the optical systemincludes components associated with four optical channels.

Embodiment 108

The system of any of Embodiments 89-107, wherein the optical systemincludes components associated with five optical channels.

Embodiment 109

The system of any of Embodiments 89-108, wherein the optical systemincludes components associated with six optical channels.

Embodiment 110

The system of any of Embodiments 89-109, wherein the optical systemincludes components associated with seven optical channels.

Embodiment 111

The system of any of Embodiments 89-110, wherein the optical systemincludes components associated with eight optical channels.

Embodiment 112

The system of any of Embodiments 89-111, wherein the optical systemincludes components associated with ten optical channels.

Embodiment 113

The system of any of Embodiments 89-112, wherein the optical systemincludes components associated with twelve optical channels.

Embodiment 114

The system of any of Embodiments 89-113, wherein each of the at leasttwo detector arrays is configured to image the same portion of the firstreference source so as to consistently provide a common first referencetemperature and wherein each of the at least two detector arrays isconfigured to image the same portion of the second reference source soas to consistently provide a common second reference temperature.

Embodiment 115

The system of any of Embodiments 89-114, further comprising atemperature controller configured to control the temperature of thefirst or second reference.

Embodiment 116

The system of any of Embodiments 89-115, further comprising one or moresensors configured to measure a temperature of the first or the secondreference.

Embodiment 117

The system of any of Embodiments 89-116, wherein the one or more sensorsare configured to communicate the measured temperature of the first orthe second reference to a temperature controller.

Embodiment 118

The system of any of Embodiments 89-117, wherein the one or more sensorsare configured to communicate the measured temperature of the first orthe second reference to the data-processing unit.

Embodiment 119

The system of any of Embodiments 89-118, wherein the first or the secondreference is associated with a heater.

Embodiment 120

The system of any of Embodiments 89-119, wherein the first or the secondreference is associated with a cooler.

Embodiment 121

The system of any of Embodiments 89-120, wherein the data-processingunit comprises processing electronics.

Embodiment 122

The system of any of Embodiments 89-121, wherein the data-processingunit comprises a processor.

Embodiment 123

The system of any of Embodiments 89-122, wherein the data-processingunit comprises one or more processors.

Embodiment 124

An infrared (IR) imaging system, the imaging system comprising:

-   -   an optical system including components associated with at least        two optical channels, said at least two optical channels being        spectrally different from one another, each of the at least two        optical channels positioned to transfer IR radiation incident on        the optical system towards a plurality of cameras;    -   at least one calibration surface with unknown temperature and        imaged by each of the at least two detectors; and    -   a data-processing unit configured to:    -   acquire a plurality of image frames with the plurality of        cameras including the imaged surface; and    -   adjust one or more parameters of the cameras in the plurality of        cameras such that a temperature estimate of the calibration        surface of the cameras in the plurality of cameras agree with        each other.

Embodiment 125

The system of Embodiment 124, wherein the one or more parameters isassociated with a gain of the cameras in the plurality of cameras.

Embodiment 126

The system of any of Embodiments 124-125, wherein the one or moreparameters is associated with a gain offset of the cameras in theplurality of cameras.

Embodiment 127

The system of any of Embodiments 124-126, wherein the calibrationsurface is displaced away from a conjugate image plane of the pluralityof cameras such that an image of the surface is defocused.

Embodiment 128

The system of any of Embodiments 124-127, wherein the calibrationsurface is positioned at a conjugate image plane of the plurality ofcameras such that an image of the surface is focused.

Embodiment 129

The system of any of Embodiments 124-128, further comprising at leastone reflecting element configured to image the surface onto theplurality of cameras.

Embodiment 130

The system of any of Embodiments 124-129, further comprising a first anda second temperature-controlled element removably positioned to block IRradiation incident on the optical system from reaching the plurality ofcameras.

Embodiment 131

The system of any of Embodiments 124-130, wherein the optical systemincludes components associated with three optical channels.

Embodiment 132

The system of any of Embodiments 124-131, wherein the optical systemincludes components associated with four optical channels.

Embodiment 133

The system of any of Embodiments 124-132, wherein the optical systemincludes components associated with six optical channels.

Embodiment 134

The system of any of Embodiments 124-133, wherein the optical systemincludes components associated with eight optical channels.

Embodiment 135

The system of any of Embodiments 124-134, wherein the optical systemincludes components associated with ten optical channels.

Embodiment 136

The system of any of Embodiments 124-135, wherein the optical systemincludes components associated with twelve optical channels.

Embodiment 137

The system of any of Embodiments 124-136, wherein the data-processingunit comprises processing electronics.

Embodiment 138

The system of any of Embodiments 124-137, wherein the data-processingunit comprises a processor.

Embodiment 139

The system of any of Embodiments 124-138, wherein the data-processingunit comprises one or more processors.

Embodiment 140

The system of any of Embodiments 124-139, wherein the calibrationsurface comprises a sidewall of a housing of the system.

Embodiment 141

An infrared (IR) imaging system, the imaging system comprising:

-   -   at least four spatially and spectrally different optical        channels configured to receive IR radiation from a common        object, each of the at least four spatially and spectrally        different optical channels comprising at least one imaging lens        configured to image the object on a Focal Plane Array (FPA)        unit; and    -   processing electronics in communication with the FPA unit,    -   wherein said infrared system is configured to:    -   acquire multispectral optical data from the at least four        different optical channels; and    -   process the multispectral optical data to detect one or more        target species present in the object.

Embodiment 142

The system of Embodiment 141, further comprising twelve opticalchannels.

Embodiment 143

The system of any of Embodiments 141-142, configured to simultaneouslyacquire multispectral optical data from the at least four differentoptical channels.

Embodiment 144

The system of any of Embodiments 141-143, further comprising a pluralityof optical filters associated with the optical channels.

Embodiment 145

The system of any of Embodiments 141-144, wherein a number of opticalfilters is two.

Embodiment 146

The system of any of Embodiments 141-145, wherein a number of opticalfilters is three.

Embodiment 147

The system of any of Embodiments 141-146, wherein a number of opticalfilters is four.

Embodiment 148

The system of any of Embodiments 141-147, wherein the plurality ofoptical filters comprise at least one long pass (LP) filter.

Embodiment 149

The system of any of Embodiments 141-148, wherein the plurality ofoptical filters comprise multiple long pass (LP) filters.

Embodiment 150

The system of any of Embodiments 141-149, wherein the plurality ofoptical filters comprise at least one short pass (SP) filter.

Embodiment 151

The system of any of Embodiments 141-150, wherein the plurality ofoptical filters comprise multiple short pass (SP) filters.

Embodiment 152

The system of any of Embodiments 141-151, wherein the plurality ofoptical filters comprise at least one band pass (BP) filter.

Embodiment 153

The system of any of Embodiments 141-152, wherein the plurality ofoptical filters comprise at one short pass (SP) filter and one long pass(LP) filter.

Embodiment 154

The system of any of Embodiments 141-153, wherein the FPA unit comprisesa plurality of FPAs.

Embodiment 155

The system of any of Embodiments 141-154, further comprising first andsecond temperature-controlled elements removably positioned to block IRradiation incident on the imaging system from reaching the FPA unit.

Embodiment 156

The system of any of Embodiments 141-155, further comprising a fieldreference configured for dynamically calibrating a plurality of the FPAsin the FPA unit.

Embodiment 157

The system of any of Embodiments 141-156, wherein the field reference isconfigured to obscure a peripheral region of an image generated by aplurality of the FPAs in the FPA unit.

Embodiment 158

The system of any of Embodiments 141-157, configured to compare spectraldata in at least one of the four optical channels acquired at a firstinstant of time with spectral data in the at least one of the fouroptical channels acquired at a second instant of time to generate atemporal difference image.

Embodiment 159

The system of any of Embodiments 141-158, configured to use a differencebetween the multispectral optical data acquired by the two opticalchannels to correct parallax-induced imaging errors.

Embodiment 160

The system of any of Embodiments 141-159, configured to use a differencebetween the multispectral optical data acquired by the two opticalchannels to estimate a distance between the system and the object.

Embodiment 161

The system of any of Embodiments 141-160, configured to estimate a sizeof the object based on the estimated distance and an opticalmagnification factor of the two optical channels.

Embodiment 162

The system of any of Embodiments 141-161, configured to compare spectraldata in one of the at least four optical channels with spectral data inanother one of the at least four optical channels to generate a spectraldifference image.

Embodiment 163

The system of any of Embodiments 141-162, further comprising a visiblelight imaging sensor.

Embodiment 164

The system of any of Embodiments 141-163, configured to use the visiblelight imaging sensor to compensate for motion-induced imaging errors.

Embodiment 165

The system of any of Embodiments 141-164, configured to process themultispectral optical data by cross-correlating multispectral opticaldata from at least one of the optical channels with a referencespectrum.

Embodiment 166

The system of any of Embodiments 141-165, configured to process themultispectral optical data using spectral unmixing.

Embodiment 167

The system of any of Embodiments 141-166, further comprising fiveoptical channels.

Embodiment 168

The system of any of Embodiments 141-167, further comprising six opticalchannels.

Embodiment 169

The system of any of Embodiments 141-168, further comprising sevenoptical channels.

Embodiment 170

The system of any of Embodiments 141-169, further comprising eightoptical channels.

Embodiment 171

The system of any of Embodiments 141-170, further comprising nineoptical channels.

Embodiment 172

The system of any of Embodiments 141-171, further comprising ten opticalchannels.

Embodiment 173

The system of any of Embodiments 141-172, further comprising elevenoptical channels.

Embodiment 174

The system of any of Embodiments 141-173, wherein a number of opticalfilters is five.

Embodiment 175

The system of any of Embodiments 141-174, wherein a number of opticalfilters is six.

Embodiment 176

The system of any of Embodiments 141-175, wherein a number of opticalfilters is seven.

Embodiment 177

The system of any of Embodiments 141-176, wherein a number of opticalfilters is eight.

Embodiment 178

The system of any of Embodiments 141-177, wherein a number of opticalfilters is nine.

Embodiment 179

The system of any of Embodiments 141-178, wherein a number of opticalfilters is ten.

Embodiment 180

The system of any of Embodiments 141-179, wherein a number of opticalfilters is eleven.

Embodiment 181

The system of any of Embodiments 141-180, wherein a number of opticalfilters is twelve.

Embodiment 182

The system of any of Embodiments 141-181, wherein the processingelectronics comprises one or more processors.

Embodiment 183

The system of any of Embodiments 141-182, further comprising a thermalreference configured to be imaged onto the FPA unit such that aplurality of frames of the acquired multispectral optical data has animage of the thermal reference source.

Embodiment 184

The system of any of Embodiments 141-183, wherein the thermal referencehas a known temperature.

Embodiment 185

The system of any of Embodiments 141-184, wherein the thermal referenceis a temperature-controlled reference source.

Embodiment 186

The system of any of Embodiments 141-185, wherein thetemperature-controlled reference source includes a heater.

Embodiment 187

The system of any of Embodiments 141-186, wherein thetemperature-controlled reference source includes a cooler.

Embodiment 188

The system of any of Embodiments 141-187, further comprising a mirrorconfigured to image the thermal reference onto the FPA unit.

Embodiment 189

The system of any of Embodiments 141-188, wherein a temperature of thethermal reference is unknown.

Embodiment 190

The system of any of Embodiments 141-189, wherein the thermal referenceis a surface.

Embodiment 191

The system of any of Embodiments 141-190, wherein the surface comprisesa wall of a housing of the system.

Embodiment 192

The system of any of Embodiments 141-191, wherein different opticalchannels receive IR radiation from the same portion of the thermalreference so as to consistently provide a common reference temperature.

Embodiment 193

The system of any of Embodiments 141-192, wherein a temperature of thesame portion of the thermal reference is unknown.

Embodiment 194

The system of any of Embodiments 141-193, configured to acquire themultispectral optical data at a frame rate between about 5 Hz to about200 Hz.

Embodiment 195

The system of any of Embodiments 141-194, wherein one or more of the atleast four optical channels is configured to collect IR radiation toprovide spectral data corresponding to a discrete spectral band locatedin the wavelength range between about 7.9 μm and about 8.4 μm.

Embodiment 196

The system of any of Embodiments 141-195, wherein each optical channelis configured to transfer spectrally distinct, two-dimensional imagedata of the common object to one or more imaging sensors.

Embodiment 196

The system of any of Embodiments 1-24, wherein the one or more sensorsare configured to communicate the measured temperature of the at leastone reference source to a temperature controller.

Embodiment 197

The system of any of Embodiments 1-24, wherein the one or more sensorsare configured to communicate the measured temperature of the at leastone reference source to the data-processing unit.

Embodiment 198

The system of any of Embodiments 26-42, wherein the one or more sensorsare configured to communicate measured temperature of the first or thesecond reference to a temperature controller.

Embodiment 199

The system of any of Embodiments 26-42, wherein the one or more sensorsare configured communicate the measured temperature of the first or thesecond reference to the data-processing unit.

Embodiment 200

An infrared (IR) imaging system for imaging a target species in anobject, the imaging system comprising:

-   -   an optical system comprising an optical focal plane array (FPA)        unit and defining a plurality of spatially and spectrally        different optical channels to transfer IR radiation towards the        optical FPA unit, each optical channel positioned to transfer a        portion of the IR radiation incident on the optical system from        the object towards the optical FPA unit; and    -   a programmable processor configured to execute instructions        stored in a tangible, non-transitory computer-readable storage        medium, to acquire, in a single occurrence of data acquisition,        multispectral optical data representing in spatial (x, y) and        spectral (λ) coordinates said object and said target species        from the IR radiation received at the optical FPA unit.

Embodiment 201

The system of Embodiment 200, wherein the multispectral data comprises anumber of spectrally different images of the object obtained from IRimage data transferred to the optical FPA unit by a correspondingoptical channel.

Embodiment 202

The system of any of Embodiments 200-201, further comprising an opticalfilter corresponding to a particular optical channel and configured totransmit the portion of IR radiation towards the optical FPA unit.

Embodiment 203

The system of any of Embodiments 200-202, wherein the optical filterincludes one of a longpass optical filter and a shortpass opticalfilter.

Embodiment 204

The system of any of Embodiments 200-203, further comprising one or morefront objective lenses.

Embodiment 205

The system of any of Embodiments 200-204, wherein the optical systemcomprises a plurality of lenses, each lens corresponding to an opticalchannel.

Embodiment 206

The system of any of Embodiments 200-205, wherein each optical channelis defined at least in part by a corresponding filter and acorresponding lens.

Embodiment 207

The system of any of Embodiments 200-206, wherein the plurality oflenses comprises a lens array.

Embodiment 208

The system of any of Embodiments 200-207, further comprising a pluralityof relay lenses configured to relay the IR radiation along the opticalchannels.

Embodiment 209

The system of any of Embodiments 200-208, further comprising a pluralityof moveable temperature-controlled reference source removably positionedto block IR radiation incident onto the optical system from reaching theoptical FPA unit.

Embodiment 210

The system of any of Embodiments 200-209, wherein the multispectraloptical data from the plurality of optical channels is capturedsubstantially simultaneously by the optical FPA unit.

Embodiment 211

The system of any of Embodiments 200-210, wherein the multispectraloptical data from the plurality of optical channels is captured duringone image frame acquisition by the optical FPA unit.

Embodiment 212

The system of any of Embodiments 200-211, further comprising first andsecond temperature-controlled moveable shutters removably positioned toblock IR radiation incident onto the optical system from reaching theoptical FPA unit.

Embodiment 213

The system of any of Embodiments 200-212, wherein the optical FPA unitis devoid of a cooling device.

Embodiment 214

The system of any of Embodiments 200-213, further comprising a filterarray.

Embodiment 215

The system of any of Embodiments 200-214, wherein the processor isconfigured to execute instructions stored in a tangible, non-transitorycomputer-readable storage medium to acquire said optical data from thetwo or more sets of imaging data.

Embodiment 216

The system of any of Embodiments 200-215, wherein the processor isconfigured to execute instructions stored in a tangible, non-transitorycomputer-readable storage medium to process the acquired optical data tocompensate for at least one of (i) parallax-induced differences betweenthe two or more sets of imaging data and (ii) difference between the twoor more sets of imaging data induced by changes in the object that arenot associated with the target species.

Embodiment 217

The system of any of Embodiments 200-216, wherein the processor isconfigured to execute instructions stored in a tangible, non-transitorycomputer-readable storage medium to process the acquired optical data togenerate a temporal reference image.

Embodiment 218

The system of any of Embodiments 200-217, wherein the processor isconfigured to execute instructions stored in a tangible, non-transitorycomputer-readable storage medium to use the temporal reference image togenerate a temporal difference image.

Embodiment 219

The system of any of Embodiments 200-218, wherein the processor isconfigured to execute instructions stored in a tangible, non-transitorycomputer-readable storage medium to process the acquired optical data toestimate a volume of a gas cloud.

Embodiment 220

The system of any of Embodiments 200-219, wherein IR radiation measuredat a pixel comprises a spectrum comprising a sum of component spectra,and wherein the processor is configured to execute instructions storedin a tangible, non-transitory computer-readable storage medium to unmixthe spectrum.

Embodiment 221

The system of any of Embodiments 200-220, wherein the optical FPA unitincludes a bolometer configured to operate without being cooled.

Embodiment 222

The system of any of Embodiments 200-221, further comprising a fieldreference for dynamically adjusting data output from the optical FPAunit.

Embodiment 223

The system of any of Embodiments 200-222, wherein the field referencecomprises an array of field stops.

Embodiment 224

The system of any of Embodiments 200-223, wherein the field referencecomprises a uniform temperature across its surface.

Embodiment 225

The system of any of Embodiments 200-224, wherein the field reference isadapted to obscure or block a peripheral portion of the IR radiationpropagating from the object towards the optical FPA unit.

Embodiment 226

The system of any of Embodiments 200-225, further comprising a visiblelight imaging sensor.

Embodiment 227

The system of any of Embodiments 200-226, wherein the processor isconfigured to execute instructions stored in a tangible, non-transitorycomputer-readable storage medium to process data received from animaging sensor to compensate for motion-induced imaging errors.

Embodiment 228

The system of any of Embodiments 200-227, wherein the processor isconfigured to execute instructions stored in a tangible, non-transitorycomputer-readable storage medium to process data received from thevisible light imaging sensor to compensate for motion-induced imagingerrors.

Embodiment 229

The system of any of Embodiments 200-228, wherein the processor isconfigured to execute instructions stored in a tangible, non-transitorycomputer-readable storage medium to construct, in the single occurrenceof data acquisition, a multispectral data cube of the object, themultispectral data cube comprising a number of spectrally differentimages of the object, each spectrally different image comprising IRimage data transferred to the optical FPA unit by a correspondingoptical channel.

Embodiment 230

The system of any of Embodiments 200-229, wherein the portion of the IRradiation corresponds to a region of wavelengths of the spectrum ofwavelengths, the region of wavelengths at least partially overlappinganother region of wavelengths transferred by another optical channel.

Embodiment 231

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of spatially and spectrally different optical        channels, at least some of the plurality of optical channels        configured to receive IR radiation from a common object, each of        the plurality of spatially and spectrally different optical        channels comprising at least one imaging lens configured to        image the object on a Focal Plane Array (FPA) unit; and    -   processing electronics in communication with the FPA unit,    -   wherein said infrared system is configured to:    -   acquire multispectral optical data from the plurality of        different optical channels; and    -   process the multispectral optical data to detect one or more        target species present in the object, and    -   wherein the system is further configured to compare spectral        data in at least one of the plurality of optical channels        acquired at a first instant of time with spectral data in at        least one of the plurality of optical channels acquired at a        second instant of time to generate a temporal difference image.

Embodiment 232

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of spatially and spectrally different optical        channels, at least some of the plurality of optical channels        configured to receive IR radiation from a common object, each of        the plurality of spatially and spectrally different optical        channels comprising at least one imaging lens configured to        image the object on a Focal Plane Array (FPA) unit; and    -   processing electronics in communication with the FPA unit,    -   wherein said infrared system is configured to:    -   acquire multispectral optical data from the plurality of        different optical channels; and    -   process the multispectral optical data to detect one or more        target species present in the object, and    -   wherein the system is configured to use a difference between the        multispectral optical data acquired by two optical channels to        correct parallax-induced imaging errors.

Embodiment 232

The system of Embodiment 231, wherein spectral characteristics of thetwo optical channels are identical.

Embodiment 233

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of spatially and spectrally different optical        channels, at least some of the plurality of optical channels        configured to receive IR radiation from a common object, each of        the plurality of spatially and spectrally different optical        channels comprising at least one imaging lens configured to        image the object on a Focal Plane Array (FPA) unit; and    -   processing electronics in communication with the FPA unit,    -   wherein said infrared system is configured to:    -   acquire multispectral optical data from the plurality of        different optical channels; and    -   process the multispectral optical data to detect one or more        target species present in the object, and    -   wherein the system is configured to use a difference between the        multispectral optical data acquired by two optical channels to        estimate a distance between the system and the object.

Embodiment 234

The system of Embodiment 233, wherein spectral characteristics of thetwo optical channels are identical.

Embodiment 235

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of spatially and spectrally different optical, at        least some of the plurality of optical channels configured to        receive IR radiation from a common object, each of the plurality        of spatially and spectrally different optical channels        comprising at least one imaging lens configured to image the        object on a Focal Plane Array (FPA) unit;    -   a visible light imaging sensor; and    -   processing electronics in communication with the FPA unit,    -   wherein said infrared system is configured to:    -   acquire multispectral optical data from the plurality of        different optical channels; and    -   process the multispectral optical data to detect one or more        target species present in the object.

Embodiment 236

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of spatially and spectrally different optical, at        least some of the plurality of optical channels configured to        receive IR radiation from a common object, each of the plurality        of spatially and spectrally different optical channels        comprising at least one imaging lens configured to image the        object on a Focal Plane Array (FPA) unit; and    -   processing electronics in communication with the FPA unit,    -   wherein said infrared system is configured to:    -   acquire multispectral optical data from the plurality of        different optical channels; and    -   process the multispectral optical data to detect one or more        target species present in the object, and    -   wherein the system is configured to compensate for        motion-induced imaging errors.

Embodiment 237

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of spatially and spectrally different, at least some        of the plurality of optical channels configured to receive IR        radiation from a common object, each of the plurality of        spatially and spectrally different optical channels comprising        at least one imaging lens configured to image the object on a        Focal Plane Array (FPA) unit; and    -   processing electronics in communication with the FPA unit,    -   wherein said infrared system is configured to:    -   acquire multispectral optical data from the plurality of        different optical channels; and    -   process the multispectral optical data to detect one or more        target species present in the object by cross-correlating        multispectral optical data from at least one of the optical        channels with a reference spectrum.

Embodiment 238

An infrared (IR) imaging system, the imaging system comprising:

-   -   a plurality of spatially and spectrally different optical, at        least some of the plurality of optical channels configured to        receive IR radiation from a common object, each of the plurality        of spatially and spectrally different optical channels        comprising at least one imaging lens configured to image the        object on a Focal Plane Array (FPA) unit; and    -   processing electronics in communication with the FPA unit,    -   wherein said infrared system is configured to:    -   acquire multispectral optical data from the plurality of        different optical channels; and    -   process the multispectral optical data to detect one or more        target species present in the object by using spectral unmixing.

Embodiment 239

An infrared (IR) imaging system, the imaging system comprising:

-   -   an optical system including an optical focal plane array (FPA)        unit, the optical system includes components associated with at        least two optical channels, said at least two optical channels        being spatially and spectrally different from one another, each        of the at least two optical channels positioned to transfer IR        radiation incident on the optical system towards the optical FPA        unit, the optical FPA unit comprising at least two detector        arrays disposed at a distance from two corresponding focusing        lenses;    -   at least one thermal reference having a known temperature,        wherein one of the at least two detector arrays is configured to        image the at least one reference; and    -   a data-processing unit, said data-processing unit configured to:    -   acquire a plurality of frames with one of the at least two        detector arrays having regions in the plurality of image frames        that correspond to the image of the reference; and    -   dynamically calibrate another of the at least two detector array        to match a temperature estimate of another of the at least two        detector array with the temperature estimate of one of the at        least two detector array.

Embodiment 240

The system of any of Embodiments 1-59, wherein the plurality of camerasare configured to acquire multispectral image data from an objectcontinuously for a duration of time.

Embodiment 241

The system of any of Embodiments 1-59, comprising at least twospectrally and spatially distinct optical channels configured totransfer two-dimensional image data of an object to the plurality ofcameras.

Details of one or more implementations of the subject matter describedin this disclosure are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages will becomeapparent from the description, the drawings and the claims. Note thatthe relative dimensions of the following figures may not be drawn toscale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of an imaging system including a common frontobjective lens that has a pupil divided spectrally and re-imaged with aplurality of lenses onto an infrared FPA.

FIG. 2 shows an embodiment with a divided front objective lens and anarray of infrared sensing FPAs.

FIG. 3A represents an embodiment employing an array of front objectivelenses operably matched with the re-imaging lens array. FIG. 3Billustrates a two-dimensional array of optical components correspondingto the embodiment of FIG. 3A.

FIG. 4 is a diagram of the embodiment employing an array of fieldreferences (e.g., field stops that can be used as references forcalibration) and an array of respectively corresponding relay lenses.

FIG. 5A is a diagram of a 4-by-3 pupil array comprising circular opticalfilters (and IR blocking material between the optical filters) used tospectrally divide an optical wavefront imaged with an embodiment of thesystem.

FIG. 5B is a diagram of a 4-by-3 pupil array comprising rectangularoptical filters (and IR blocking material between the optical filters)used to spectrally divide an optical wavefront imaged with an embodimentof the system.

FIG. 6A depicts theoretical plots of transmission characteristics of acombination of band-pass filters used with an embodiment of the system.

FIG. 6B depicts theoretical plots of transmission characteristics of aspectrally multiplexed notch-pass filter combination used in anembodiment of the system.

FIG. 6C shows theoretical plots of transmission characteristics ofspectrally multiplexed long-pass filter combination used in anembodiment of the system.

FIG. 6D shows theoretical plots of transmission characteristics ofspectrally multiplexed short-pass filter combination used in anembodiment of the system.

FIG. 7 is a set of video-frames illustrating operability of anembodiment of the system used for gas detection.

FIGS. 8A and 8B are plots (on axes of wavelength in microns versus theobject temperature in Celsius representing effective optical intensityof the object) illustrating results of dynamic calibration of anembodiment of the system.

FIGS. 9A and 9B illustrate a cross-sectional view of differentembodiments of an imaging system comprising an arrangement of referencesources and mirrors that can be used for dynamic calibration.

FIGS. 10A-10C illustrate a plan view of different embodiments of animaging system comprising an arrangement of reference sources andmirrors that can be used for dynamic calibration.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

The following description is directed to certain implementations for thepurposes of describing the innovative aspects of this disclosure.However, a person having ordinary skill in the art will readilyrecognize that the teachings herein can be applied in a multitude ofdifferent ways. The described implementations may be implemented in anydevice, apparatus, or system that can be configured to operate as animaging system such as in an infra-red imaging system. The methods andsystems described herein can be included in or associated with a varietyof devices such as, but not limited to devices used for visible andinfrared spectroscopy, multispectral and hyperspectral imaging devicesused in oil and gas exploration, refining, and transportation,agriculture, remote sensing, defense and homeland security,surveillance, astronomy, environmental monitoring, etc. The methods andsystems described herein have applications in a variety of fieldsincluding but not limited to agriculture, biology, physics, chemistry,defense and homeland security, environment, oil and gas industry, etc.The teachings are not intended to be limited to the implementationsdepicted solely in the Figures, but instead have wide applicability aswill be readily apparent to one having ordinary skill in the art.

Various embodiments disclosed herein describe a divided-apertureinfrared spectral imaging (DAISI) system that is structured and adaptedto provide identification of target chemical contents of the imagedscene. The system is based on spectrally-resolved imaging and canprovide such identification with a single-shot (also referred to as asnapshot) comprising a plurality of images having different wavelengthcompositions that are obtained generally simultaneously. Without anyloss of generality, snapshot refers to a system in which most of thedata elements that are collected are continuously viewing the lightemitted from the scene. In contrast in scanning systems, at any giventime only a minority of data elements are continuously viewing a scene,followed by a different set of data elements, and so on, until the fulldataset is collected. Relatively fast operation can be achieved in asnapshot system because it does not need to use spectral or spatialscanning for the acquisition of infrared (IR) spectral signatures of thetarget chemical contents. Instead, IR detectors (such as, for example,infrared focal plane arrays or FPAs) associated with a plurality ofdifferent optical channels having different wavelength profiles can beused to form a spectral cube of imaging data. Although spectral data canbe obtained from a single snapshot comprising multiple simultaneouslyacquired images corresponding to different wavelength ranges, in variousembodiments, multiple snap shots may be obtained. In variousembodiments, these multiple snapshots can be averaged. Similarly, incertain embodiments multiple snap shots may be obtained and a portion ofthese can be selected and possibly averaged. Also, in contrast tocommonly used IR spectral imaging systems, the DAISI system does notrequire cooling. Accordingly, it can advantageously use uncooledinfrared detectors. For example, in various implementations, the imagingsystems disclosed herein do not include detectors configured to becooled to a temperature below 300 Kelvin. As another example, in variousimplementations, the imaging systems disclosed herein do not includedetectors configured to be cooled to a temperature below 273 Kelvin. Asyet another example, in various implementations, the imaging systemsdisclosed herein do not include detectors configured to be cooled to atemperature below 250 Kelvin. As another example, in variousimplementations, the imaging systems disclosed herein do not includedetectors configured to be cooled to a temperature below 200 Kelvin.

Implementations disclosed herein provide several advantages overexisting IR spectral imaging systems, most if not all of which mayrequire FPAs that are highly sensitive and cooled in order tocompensate, during the optical detection, for the reduction of thephoton flux caused by spectrum-scanning operation. The highly sensitiveand cooled FPA systems are expensive and require a great deal ofmaintenance. Since various embodiments disclosed herein are configuredto operate in single-shot acquisition mode without spatial and/orspectral scanning, the instrument can receive photons from a pluralityof points (e.g., every point) of the object substantiallysimultaneously, during the single reading. Accordingly, the embodimentsof imaging system described herein can collect a substantially greateramount of optical power from the imaged scene (for example, an order ofmagnitude more photons) at any given moment in time especially incomparison with spatial and/or spectral scanning systems. Consequently,various embodiments of the imaging systems disclosed herein can beoperated using uncooled detectors (for example, FPA unit including anarray of microbolometers) that are less sensitive to photons in the IRbut are well fit for continuous monitoring applications. For example, invarious implementations, the imaging systems disclosed herein do notinclude detectors configured to be cooled to a temperature below 300Kelvin. As another example, in various implementations, the imagingsystems disclosed herein do not include detectors configured to becooled to a temperature below 273 Kelvin. As yet another example, invarious implementations, the imaging systems disclosed herein do notinclude detectors configured to be cooled to a temperature below 250Kelvin. As another example, in various implementations, the imagingsystems disclosed herein do not include detectors configured to becooled to a temperature below 200 Kelvin. Imaging systems includinguncooled detectors can be capable of operating in extreme weatherconditions, require less power, are capable of operation during day andnight, and are less expensive. Some embodiments described herein canalso be less susceptible to motion artifacts in comparison withspatially and/or spectrally scanning systems which can cause errors ineither the spectral data, spatial data, or both.

FIG. 1 provides a diagram schematically illustrating spatial andspectral division of incoming light by an embodiment 100 of a dividedaperture infrared spectral imager (DAISI) system that can image anobject 110 possessing IR spectral signature(s). The system 100 includesa front objective lens 124, an array of optical filters 130, an array ofreimaging lenses 128 and a detector array 136. In various embodiments,the detector array 136 can include a single FPA or an array of FPAs.Each detector in the detector array 136 can be disposed at the focus ofeach of the lenses in the array of reimaging lenses 128. In variousembodiments, the detector array 136 can include a plurality ofphoto-sensitive devices. In some embodiments, the plurality ofphoto-sensitive devices may comprise a two-dimensional imaging sensorarray that is sensitive to radiation having wavelengths between 1 μm and20 μm (for example, in near infra-red wavelength range, mid infra-redwavelength range, or long infra-red wavelength range,). In variousembodiments, the plurality of photo-sensitive devices can include CCD orCMOS sensors, bolometers, microbolometers or other detectors that aresensitive to infra-red radiation.

An aperture of the system 100 associated with the front objective lenssystem 124 is spatially and spectrally divided by the combination of thearray of optical filters 130 and the array of reimaging lenses 128. Invarious embodiments, the combination of the array of optical filters 130and the array of reimaging lenses 128 can be considered to form aspectrally divided pupil that is disposed forward of the opticaldetector array 136. The spatial and spectral division of the apertureinto distinct aperture portions forms a plurality of optical channels120 along which light propagates. In various embodiments, the array 128of re-imaging lenses 128 a and the array of spectral filters 130 whichrespectively correspond to the distinct optical channels 120. Theplurality of optical channels 120 can be spatially and/or spectrallydistinct. The plurality of optical channels 120 can be formed in theobject space and/or image space. In one implementation, the distinctchannels 120 may include optical channels that are separated angularlyin space. The array of spectral filters 130 may additionally include afilter-holding aperture mask (comprising, for example, IR light-blockingmaterials such as ceramic, metal, or plastic). Light from the object 110(for example a cloud of gas), the optical properties of which in the IRare described by a unique absorption, reflection and/or emissionspectrum, is received by the aperture of the system 100. This lightpropagates through each of the plurality of optical channels 120 and isfurther imaged onto the optical detector array 136. In variousimplementations, the detector array 136 can include at least one FPA. Invarious embodiments, each of the re-imaging lenses 128 a can bespatially aligned with a respectively-corresponding spectral region. Inthe illustrated implementation, each filter element from the array ofspectral filters 130 corresponds to a different spectral region. Eachre-imaging lens 128 a and the corresponding filter element of the arrayof spectral filter 130 can coincide with (or form) a portion of thedivided aperture and therefore with respectively-corresponding spatialchannel 120. Accordingly, in various embodiment an imaging lens 128 aand a corresponding spectral filter can be disposed in the optical pathof one of the plurality of optical channels 120. Radiation from theobject 110 propagating through each of the plurality of optical channels120 travels along the optical path of each re-imaging lens 128 a and thecorresponding filter element of the array of spectral filter 130 and isincident on the detector array (e.g., FPA component) 136 to form asingle image (e.g., sub-image) of the object 110. The image formed bythe detector array 136 generally includes a plurality of sub-imagesformed by each of the optical channels 120. Each of the plurality ofsub-images can provide different spatial and spectral information of theobject 110. The different spatial information results from some parallaxbecause of the different spatial locations of the smaller apertures ofthe divided aperture. In various embodiments, adjacent sub-images can becharacterized by close or substantially equal spectral signatures. Thedetector array (e.g., FPA component) 136 is further operably connectedwith a processor 150 (not shown). The processor 150 can be programmed toaggregate the data acquired with the system 100 into a spectral datacube. The data cube represents, in spatial (x, y) and spectral (λ)coordinates, an overall spectral image of the object 110 within thespectral region defined by the combination of the filter elements in thearray of spectral filters 130. Additionally, in various embodiments, theprocessor or processing electronics 150 may be programmed to determinethe unique absorption characteristic of the object 110. Also, theprocessor/processing electronics 150 can, alternatively or in addition,map the overall image data cube into a cube of data representing, forexample, spatial distribution of concentrations, c, of targeted chemicalcomponents within the field of view associated with the object 110.

Various implementations of the embodiment 100 can include an optionalmoveable temperature-controlled reference source 160 including, forexample, a shutter system comprising one or more reference shuttersmaintained at different temperatures. The reference source 160 caninclude a heater, a cooler or a temperature-controlled elementconfigured to maintain the reference source 160 at a desiredtemperature. For example, in various implementations, the embodiment 100can include two reference shutters maintained at different temperatures.The reference source 160 is removably and, in one implementation,periodically inserted into an optical path of light traversing thesystem 100 from the object 110 to the detector array (e.g., FPAcomponent) 136 along at least one of the channels 120. The removablereference source 160 thus can block such optical path. Moreover, thisreference source 160 can provide a reference IR spectrum to recalibratevarious components including the detector array 136 of the system 100 inreal time. The configuration of the moveable reference source 160 isfurther discussed below.

In the embodiment 100, the front objective lens system 124 is shown toinclude a single front objective lens positioned to establish a commonfield-of-view (FOV) for the reimaging lenses 128 a and to define anaperture stop for the whole system. In this specific case, the aperturestop substantially spatially coincides with and/or is about the samesize or slightly larger, as the plurality of smaller limiting aperturescorresponding to different optical channels 120. As a result, thepositions for spectral filters of the different optical channels 120coincide with the position of the aperture stop of the whole system,which in this example is shown as a surface between the lens system 124and the array 128 of the reimaging lenses 128 a. In variousimplementations, the lens system 124 can be an objective lens 124.However, the objective lens 124 is optional and various embodiments ofthe system 100 need not include the objective lens 124. In variousembodiments, the objective lens 124 can slightly shifts the imagesobtained by the different detectors in the array 136 spatially along adirection perpendicular to optical axis of the lens 124, thus thefunctionality of the system 100 is not necessarily compromised when theobjective lens 124 is not included. Generally, however, the fieldapertures corresponding to different optical channels may be located inthe same or different planes. These field apertures may be defined bythe aperture of the reimaging lens 128 a and/or filters in the dividedaperture 130 in certain implementations. In one implementation, thefield apertures corresponding to different optical channels can belocated in different planes and the different planes can be opticalconjugates of one another. Similarly, while all of the filter elementsin the array of spectral filters 130 of the embodiment 100 are shown tolie in one plane, generally different filter elements of the array ofspectral filter 130 can be disposed in different planes. For example,different filter elements of the array of spectral filters 130 can bedisposed in different planes that are optically conjugate to oneanother. However, in other embodiments, the different filter elementscan be disposed in non-conjugate planes.

In contrast to the embodiment 100, the front objective lens 124 need notbe a single optical element, but instead can include a plurality oflenses 224 as shown in an embodiment 200 of the DAISI imaging system inFIG. 2. These lenses 224 are configured to divide an incoming opticalwavefront from the object 110. For example, the array of front objectivelenses 224 can be disposed so as to receive an IR wavefront emitted bythe object that is directed toward the DAISI system. The plurality offront objective lenses 224 divide the wavefront spatially intonon-overlapping sections. FIG. 2 shows three objective lenses 224 in afront optical portion of the optical system contributing to the spatialdivision of the aperture of the system in this example. The plurality ofobjective lenses 224, however, can be configured as a two-dimensional(2D) array of lenses. FIG. 2 presents a general view of the imagingsystem 200 and the resultant field of view of the imaging system 200. Anexploded view 202 of the imaging system 200 is also depicted in greaterdetail in a figure inset of FIG. 2. As illustrated in the detailed view202, the embodiment of the imaging system 200 includes a field reference204 at the front end of the system. The field reference 204 can be usedto truncate the field of view. The configuration illustrated in FIG. 2has an operational advantage over embodiment 100 of FIG. 1 in that theoverall size and/or weight and/or cost of manufacture of the embodiment200 can be greatly reduced because the objective lens is smaller. Eachpair of the lenses in the array 224 and the array 128 is associated witha field of view (FOV). Each pair of lenses in the array 224 and thearray 128 receives light from the object from a different angle.Accordingly, the FOV of the different pairs of lenses in the array 224and the array 128 do not completely overlap as a result of parallax. Asthe distance between the imaging system 200 (portion 202) and the object110 increases, the overlapping region 230 between the FOVs of theindividual lenses 224 increases while the amount of parallax 228 remainsapproximately the same, thereby reducing its effect on the system 200.When the ratio of the parallax-to-object-distance is substantially equalto the pixel-size-to-system-focal-length ratio then the parallax effectmay be considered to be negligible and, for practical purposes, nolonger distinguishable. While the lenses 224 are shown to be disposedsubstantially in the same plane, optionally different objective lensesin the array of front objective lenses 224 can be disposed in more thanone plane. For example, some of the individual lenses 224 can bedisplaced with respect to some other individual lenses 224 along theaxis 226 (not shown) and/or have different focal lengths as compared tosome other lenses 224. As discussed below, the field reference 204 canbe useful in calibrating the multiple detectors 236.

In one implementation, the front objective lens system such as the arrayof lenses 224 is configured as an array of lenses integrated or moldedin association with a monolithic substrate. Such an arrangement canreduce the costs and complexity otherwise accompanying the opticaladjustment of individual lenses within the system. An individual lens224 can optionally include a lens with varying magnification. As oneexample, a pair of thin and large diameter Alvarez plates can be used inat least a portion of the front objective lens system. Without any lossof generality, the Alvarez plates can produce a change in focal lengthwhen translated orthogonally with respect to the optical beam.

In further reference to FIG. 1, the detector array 136 (e.g., FPAcomponent) configured to receive the optical data representing spectralsignature(s) of the imaged object 110 can be configured as a singleimaging array (e.g., FPA) 136. This single array may be adapted toacquire more than one image (formed by more than one optical channel120) simultaneously. Alternatively, the detector array 136 may include aFPA unit. In various implementations, the FPA unit can include aplurality of optical FPAs. At least one of these plurality of FPAs canbe configured to acquire more than one spectrally distinct image of theimaged object. For example, as shown in the embodiment 200 of FIG. 2, invarious embodiments, the number of FPAs included in the FPA unit maycorrespond to the number of the front objective lenses 224. In theembodiment 200 of FIG. 2, for example, three FPAs 236 are providedcorresponding to the three objective lenses 224. In one implementationof the system, the FPA unit can include an array of microbolometers. Theuse of multiple microbolometers advantageously allows for an inexpensiveway to increase the total number of detection elements (i.e. pixels) forrecording of the three-dimensional data cube in a single acquisitionevent (i.e. one snapshot). In various embodiments, an array ofmicrobolometers more efficiently utilizes the detector pixels of thearray of FPAs (e.g., each FPA) as the number of unused pixels isreduced, minimized and/or eliminated between the images that may existwhen using a single microbolometer.

FIG. 3A illustrates schematically an embodiment 300 of the imagingsystem in which the number of the front objective lenses 324 a in thelens array 324, the number of re-imaging lenses 128 a in the lens array128, and the number of FPAs 336 are the same. So configured, eachcombination of respectively corresponding front objective lens 324,re-imaging lens 128 a, and FPAs 336 constitutes an individual imagingchannel. Such a channel is associated with acquisition of the IR lighttransmitted from the object 110 through an individual filter element ofthe array of optical filters 130. A field reference 338 of the system300 is configured to have a uniform temperature across its surface andbe characterized by a predetermined spectral curve of radiationemanating therefrom. In various implementations, the field reference 338can be used as a calibration target to assist in calibrating ormaintaining calibration of the FPA. Accordingly, in variousimplementations, the field reference 338 is used for dynamicallyadjusting the data output from each FPA 336 after acquisition of lightfrom the object 110. This dynamic calibration process helps provide thatoutput of the different (e.g., most, or each of the) FPA 336 representscorrect acquired data, with respect to the other FPAs 336 for analysis,as discussed below in more detail.

FIG. 3B illustrates the plan view perpendicular to the axis 226 of anembodiment 300 of the imaging system illustrated in FIG. 3A. For theembodiment shown in FIG. 3B, the optical components (e.g., objectivelenses 324 a, filter elements of the array of spectral filters 130,re-imaging lenses 128 a and FPA units 336) are arranged as a 4×3 array.In one implementation, the 4×3 array 340 of optical components (lenses324 a, 128 a; detector elements 336) is used behind the temperaturecontrolled reference target 160. The field reference aperture 338 can beadapted to obscure and/or block a peripheral portion of the bundle oflight propagating from the object 110 towards the FPA units 336. As aresult, the field reference 338 obscures and/or blocks the border orperipheral portion(s) of the images of the object 110 formed on the FPAelements located along the perimeter 346 of the detector system.Generally, two elements of the FPA unit will produce substantially equalvalues of digital counts when they are used to observe the same portionof the scene in the same spectral region using the same optical train.If any of these input parameters (for example, scene to be observed,spectral content of light from the scene, or optical elements deliveringlight from the scene to the two detector elements) differ, the countsassociated with the elements of the FPA unit will differ as well.Accordingly, and as an example, in a case when the two FPAs of the FPAunit 336 (such as those denoted as #6 and #7 in FIG. 3B) remainsubstantially un-obscured by the field reference 338, the outputs fromthese FPAs can be dynamically adjusted to the output from one of theFPAs located along perimeter 346 (such as, for example, the FPA element#2 or FPA element #11) that processes light having similar spectralcharacteristics.

FIG. 4 illustrates schematically a portion of another embodiment of animaging system 400 that contains an array 424 of front objective lenses424 a. The array 424 of lenses 424 a adapted to receive light from theobject 110 and relay the received light to the array 128 of re-imaginglenses 128 a through an array 438 of field references (or field stops)438 a, and through an array 440 of the relay lenses. The spectralcharacteristics of the field references/field stops 438 a can be known.The field references 438 a are disposed at corresponding intermediateimage planes defined, with respect to the object 110, by respectivelycorresponding front objective lenses 424 a. When refractivecharacteristics of all of the front objective lenses 424 a aresubstantially the same, all of the field references 438 a are disposedin the same plane. A field reference 438 a of the array 438 obscures (orcasts a shadow on) a peripheral region of a corresponding image (e.g.,sub-image) formed at the detector plane 444 through a respectivelycorresponding spatial imaging channel 450 of the system 400 prior tosuch image being spectrally processed by the processor 150. The array440 of relay lenses then transmits light along each of the imagingchannels 450 through different spectral filters 454 a of the filterarray 454, past the calibration apparatus that includes two temperaturecontrolled shutters 460 a, 460 b, and then onto the detector module 456.In various embodiments, the detector module 456 can include amicrobolometer array or some other IR FPA.

The embodiment 400 has several operational advantages. It is configuredto provide a spectrally known object within every image (e.g.,sub-image) and for every snapshot acquisition which can be calibratedagainst. Such spectral certainty can be advantageous when using an arrayof IR FPAs like microbolometers, the detection characteristics of whichcan change from one imaging frame to the next due to, in part, changesin the scene being imaged as well as the thermal effects caused byneighboring FPAs. In various embodiments, the field reference array 438of the embodiment 400—can be disposed within the Rayleigh range(approximately corresponding to the depth of focus) associated with thefront objective lenses 424, thereby removing unusable blurred pixels dueto having the field reference outside of this range. Additionally, theembodiment 400 of FIG. 4 can be more compact than, for example, theconfiguration 300 of FIG. 3A. In the system shown in FIG. 3A, forexample, the field reference 338 may be separated from the lens array324 by a distance greater than several (for example, five) focal lengthsto minimize/reduce blur contributed by the field reference to an imageformed at a detector plane.

In various embodiments, the multi-optical FPA unit of the IR imagingsystem can additionally include an FPA configured to operate in avisible portion of the spectrum. In reference to FIG. 1, for example, animage of the scene of interest formed by such visible-light FPA may beused as a background to form a composite image by overlapping an IRimage with the visible-light image. The IR image may be overlappedvirtually, with the use of a processor and specifically-designedcomputer program product enabling such data processing, or actually, bya viewer. The IR image may be created based on the image data acquiredby the individual FPAs 136. The so-formed composite image facilitatesthe identification of the precise spatial location of the targetspecies, the spectral signatures of which the system is able todetect/recognize.

Optical Filters.

The optical filters, used with an embodiment of the system, that definespectrally-distinct IR image (e.g., sub-image) of the object can employabsorption filters, interference filters, and Fabry-Perot etalon basedfilters, to name just a few. When interference filters are used, theimage acquisition through an individual imaging channel defined by anindividual re-imaging lens (such as a lens 128 a of FIGS. 1, 2, 3, and4) may be carried out in a single spectral bandwidth or multiplespectral bandwidths. Referring again to the embodiments 100, 200, 300,400 of FIGS. 1 through 4, and in further reference to FIG. 3B, examplesof a 4-by-3 array of spectral filters 130 is shown in FIGS. 5A and 5B.Individual filters 1 through 12 are juxtaposed with a supportingopto-mechanical element (not shown) to define a filter-array plane thatis oriented, in operation, substantially perpendicularly to the generaloptical axis 226 of the imaging system. In various implementations, theindividual filters 1 through 12 need not be discrete optical components.Instead, the individual filters 1 through 12 can comprise one or morecoatings that are applied to one or more surfaces of the reimaginglenses (such as a lens 128 a of FIGS. 1, 2, 3, and 4) or the surfaces ofone or more detectors.

The optical filtering configuration of various embodiments disclosedherein may advantageously use a bandpass filter defining a specifiedspectral band. Any of the filters 0a through 3a the transmission curvesof which are shown in FIG. 6A may, for example, be used. The filters maybe placed in front of the optical FPA (or generally, between the opticalFPA and the object). In particular, and in further reference to FIGS. 1,2 3, and 4, when optical detector arrays 136, 236, 336, 456 includemicrobolometers, the predominant contribution to noise associated withimage acquisition is due to detector noise. To compensate and/or reducethe noise, various embodiments disclosed herein utilizespectrally-multiplexed filters. In various implementations, thespectrally-multiplexed filters can comprise a plurality of long passfilters, a plurality long pass filters, a plurality of band pass filtersand any combinations thereof. An example of the spectral transmissioncharacteristics of spectrally-multiplexed filters 0b through 3d for usewith various embodiments of imaging systems disclosed herein is depictedin FIG. 6B. Filters of FIG. 6C can be referred to as long-wavelengthpass, LP filters. An LP filter generally attenuates shorter wavelengthsand transmits (passes) longer wavelengths (e.g., over the active rangeof the target IR portion of the spectrum). In various embodiments,short-wavelength-pass filters, SP, may also be used. An SP filtergenerally attenuates longer wavelengths and transmits (passes) shorterwavelengths (e.g., over the active range of the target IR portion of thespectrum). At least in part due to the snap-shot/non-scanning mode ofoperation, embodiments of the imaging system described herein can useless sensitive microbolometers without compromising the SNR. The use ofmicrobolometers, as detector-noise-limited devices, in turn not onlybenefits from the use of spectrally multiplexed filters, but also doesnot require cooling of the imaging system during normal operation.

Referring again to FIGS. 6A, 6B, 6C, and 6D, each of the filters (0b . .. 3d) transmits light in a substantially wider region of theelectromagnetic spectrum as compared to those of the filters (0a . . .3a). Accordingly, when the spectrally-multiplexed set of filters (0b . .. 0d) is used with an embodiment of the imaging system, the overallamount of light received by the FPAs (for example, 236, 336) is largerthan would be received when using the bandpass filters (0a . . . 4a).This “added” transmission of light defined by the use of thespectrally-multiplexed LP (or SP) filters facilitates an increase of thesignal on the FPAs above the level of the detector noise. Additionally,by using, in an embodiment of the imaging system, filters havingspectral bandwidths greater than those of band-pass filters, theuncooled FPAs of the embodiment of the imaging system experience lessheating from radiation incident thereon from the imaged scene and fromradiation emanating from the FPA in question itself. This reducedheating is due to a reduction in the back-reflected thermal emission(s)coming from the FPA and reflecting off of the filter from thenon-band-pass regions. As the transmission region of the multiplexed LP(or SP) filters is wider, such parasitic effects are reduced therebyimproving the overall performance of the FPA unit.

In one implementation, the LP and SP filters can be combined, in aspectrally-multiplexed fashion, in order to increase or maximize thespectral extent of the transmission region of the filter system of theembodiment.

The advantage of using spectrally multiplexed filters is appreciatedbased on the following derivation, in which a system of M filters isexamined (although it is understood that in practice an embodiment ofthe invention can employ any number of filters). As an illustrativeexample, the case of M=7 is considered. Analysis presented below relatesto one spatial location in each of the images (e.g., sub-images) formedby the differing imaging channels (e.g., different optical channels 120)in the system. A similar analysis can be performed for each point at animage (e.g., sub-image), and thus the analysis can be appropriatelyextended as required.

The unknown amount of light within each of the M spectral channels(corresponding to these M filters) is denoted with f₁, f₂, f₃, . . .f_(M), and readings from corresponding detector elements receiving lighttransmitted by each filter is denoted as g₁, g₂, g₃ . . . g_(M), whilemeasurement errors are represented by n₁, n₂, n₃, . . . n_(M). Then, thereadings at the seven FPA pixels each of which is optically filtered bya corresponding band-pass filter of FIG. 6A can be represented by:g ₁ =f ₁ +n ₁,g ₂ =f ₂ +n ₂,g ₃ =f ₃ +n ₃,g ₄ =f ₄ +n ₄,g ₅ =f ₅ +n ₅,g ₆ =f ₆ +n ₆,g ₇ =f ₇ +n ₇,

These readings (pixel measurements) g_(i) are estimates of the spectralintensities f_(i). The estimates g_(i) are not equal to thecorresponding f_(i) values because of the measurement errors n_(i).However, if the measurement noise distribution has zero mean, then theensemble mean of each individual measurement can be considered to beequal to the true value, i.e. <g_(i)>=f_(i). Here, the angle bracketsindicate the operation of calculating the ensemble mean of a stochasticvariable. The variance of the measurement can, therefore, be representedas:<(g _(i) −f _(i))² >=<n _(i) ²>=σ²

In embodiments utilizing spectrally-multiplexed filters, in comparisonwith the embodiments utilizing band-pass filters, the amount of radiantenergy transmitted by each of the spectrally-multiplexed LP or SPfilters towards a given detector element can exceed that transmittedthrough a spectral band of a band-pass filter. In this case, theintensities of light corresponding to the independent spectral bands canbe reconstructed by computational means. Such embodiments can bereferred to as a “multiplex design”.

One matrix of such “multiplexed filter” measurements includes a Hadamardmatrix requiring “negative” filters that may not be necessarilyappropriate for the optical embodiments disclosed herein. An S-matrixapproach (which is restricted to having a number of filters equal to aninteger that is multiple of four minus one) or a row-doubled Hadamardmatrix (requiring a number of filters to be equal to an integer multipleof eight) can be used in various embodiments. Here, possible numbers offilters using an S-matrix setup are 3, 7, 11, etc and, if a row-doubledHadamard matrix setup is used, then the possible number of filters is 8,16, 24, etc. For example, the goal of the measurement may be to measureseven spectral band f_(i) intensities using seven measurements g_(i) asfollows:g ₁ =f ₁+0+f ₃+0+f ₅+0+f ₇ +n ₁,g ₂=0+f ₂ +f ₃+0+0+f ₆ +f ₇ +n ₂g ₃ =f ₁ +f ₂+0+0+f ₅+0+f ₇ +n ₃g ₄=0+0+0+f ₄ +f ₅ +f ₇ +f ₈ +n ₄g ₅ =f ₁+0+f ₃ +f ₄+0+f ₆+0+n ₅g ₆=0+f ₂ +f ₃ +f ₄ +f ₅+0+0+n ₆g ₇ =f ₁ +f ₂+0+f ₄+0+0+f ₇ +n ₇

Optical transmission characteristics of the filters described above aredepicted in FIG. 6B. Here, a direct estimate of the f_(i) is no longerprovided through a relationship similar to <g_(i)>=f_(i). Instead, if a“hat” notation is used to denote an estimate of a given value, then alinear combination of the measurements can be used such as, for example,{circumflex over (f)} ₁=¼(+g ₁ −g ₂ +g ₃ −g ₄ +g ₅ −g ₆ +g ₇),{circumflex over (f)} ₂=¼(−g ₁ +g ₂ +g ₃ −g ₄ −g ₅ +g ₆ +g ₇),{circumflex over (f)} ₃=¼(+g ₁ +g ₂ −g ₃ −g ₄ +g ₅ +g ₆ −g ₇),{circumflex over (f)} ₄=¼(−g ₁ −g ₂ −g ₃ +g ₄ +g ₅ +g ₆ +g ₇),{circumflex over (f)} ₅=¼(+g ₁ −g ₂ +g ₃ +g ₄ −g ₅ +g ₆ −g ₇),{circumflex over (f)} ₆=¼(−g ₁ +g ₂ +g ₃ +g ₄ +g ₅ −g ₆ −g ₇),{circumflex over (f)} ₇=¼(+g ₁ +g ₂ −g ₃ +g ₄ −g ₅ −g ₆ +g ₇),

These {circumflex over (f)}_(i) are unbiased estimates when the n_(i)are zero mean stochastic variables, so that <{circumflex over(f)}_(i)−f_(i)>=0. The measurement variance corresponding to the i^(th)measurement is given by the equation below:<({circumflex over (f)} _(i) −f _(i))²>= 7/16σ²

From the above equation, it is observed that by employingspectrally-multiplexed system the signal-to-noise ratio (SNR) of ameasurement is improved by a factor of √{square root over (16/7)}=1.51.

For N channels, the SNR improvement achieved with aspectrally-multiplexed system can be expressed as (N+1)/(2√{square rootover (N)}). For example, an embodiment employing 12 spectral channels(N=12) is characterized by a SNR improvement, over anon-spectrally-multiplexed system, comprising a factor of up to 1.88.

Two additional examples of related spectrally-multiplexed filterarrangements 0c through 3c and 0d through 3d that can be used in variousembodiments of the imaging systems described herein are shown in FIGS.6C and 6D, respectively. The spectrally-multiplexed filters shown inFIGS. 6C and 6D can be used in embodiments of imaging systems employinguncooled FPAs (such as microbolometers). FIG. 6C illustrates a set ofspectrally-multiplexed long-wavelength pass (LP) filters used in thesystem. An LP filter generally attenuates shorter wavelengths andtransmits (passes) longer wavelengths (e.g., over the active range ofthe target IR portion of the spectrum). A single spectral channel havinga transmission characteristic corresponding to the difference betweenthe spectral transmission curves of at least two of these LP filters canbe used to procure imaging data for the data cube using an embodiment ofthe system described herein. In various implementations, the spectralfilters disposed with respect to the different FPAs can have differentspectral characteristics. In various implementations, the spectralfilters may be disposed in front of only some of the FPAs while theremaining FPAs may be configured to receive unfiltered light. Forexample, in some implementations, only 9 of the 12 detectors in the 4×3array of detectors described above may be associated with a spectralfilter while the other 3 detectors may be configured to receivedunfiltered light. Such a system may be configured to acquire spectraldata in 10 different spectral channels in a single data acquisitionevent.

The use of microbolometers, as detector-noise-limited devices, in turnnot only can benefit from the use of spectrally multiplexed filters, butalso does not require cooling of the imaging system during normaloperation. In contrast to imaging systems that include highly sensitiveFPA units with reduced noise characteristics, the embodiments of imagingsystems described herein can employ less sensitive microbolometerswithout compromising the SNR. This result is at least in part due to thesnap-shot/non-scanning mode of operation.

As discussed above, an embodiment may optionally, and in addition to atemperature-controlled reference unit (for example temperaturecontrolled shutters such as shutters 160; 160 a, 160 b, 460 a, 460 b),employ a field reference component (e.g., field reference aperture 338in FIG. 3A), or an array of field reference components (e.g., filedreference apertures 438 in FIG. 4), to enable dynamic calibration. Suchdynamic calibration can be used for spectral acquisition of one or moreor every data cube. Such dynamic calibration can also be used for aspectrally-neutral camera-to-camera combination to enable dynamiccompensation of parallax artifacts. The use of thetemperature-controlled reference unit (for example,temperature-controlled shutter system 160) and field-referencecomponent(s) facilitates maintenance of proper calibration of each ofthe FPAs individually and the entire FPA unit as a whole.

In particular, and in further reference to FIGS. 1, 2, 3, and 4, thetemperature-controlled unit generally employs a system having first andsecond temperature zones maintained at first and second differenttemperatures. For example, shutter system of each of the embodiments100, 200, 300 and 400 can employ not one but at least twotemperature-controlled shutters that are substantially parallel to oneanother and transverse to the general optical axis 226 of theembodiment(s) 100, 200, 300, 400. Two shutters at two differenttemperatures may be employed to provide more information forcalibration; for example, the absolute value of the difference betweenFPAs at one temperature as well as the change in that difference withtemperature change can be recorded. Referring, for example, to FIG. 4,in which such multi-shutter structure is shown, the use of multipleshutters enables the user to create a known reference temperaturedifference perceived by the FPAs 456. This reference temperaturedifference is provided by the IR radiation emitted by the shutter(s) 160a, 160 b when these shutters are positioned to block the radiation fromthe object 110. As a result, not only the offset values corresponding toeach of the individual FPAs pixels can be adjusted but also the gainvalues of these FPAs. In an alternative embodiment, the system havingfirst and second temperature zones may include a single or multi-portionpiece. This single or multi-portion piece may comprise for example aplate. This piece may be mechanically-movable across the optical axiswith the use of appropriate guides and having a first portion at a firsttemperature and a second portion at a second temperature.

Indeed, the process of calibration of an embodiment of the imagingsystem starts with estimating gain and offset by performing measurementsof radiation emanating, independently, from at least twotemperature-controlled shutters of known and different radiances. Thegain and offset can vary from detector pixel to detector pixel.Specifically, first the response of the detector unit 456 to radiationemanating from one shutter is carried out. For example, the firstshutter 160 a blocks the FOV of the detectors 456 and the temperature T₁is measured directly and independently with thermistors. Following suchinitial measurement, the first shutter 160 a is removed from the opticalpath of light traversing the embodiment and another second shutter (forexample, 160 b) is inserted in its place across the optical axis 226 toprevent the propagation of light through the system. The temperature ofthe second shutter 160 b can be different than the first shutter(T₂≠T₁). The temperature of the second shutter 160 b is alsoindependently measured with thermistors placed in contact with thisshutter, and the detector response to radiation emanating from theshutter 160 b is also recorded. Denoting operational response of FPApixels (expressed in digital numbers, or “counts”) as g_(i) to a sourceof radiance L_(i), the readings corresponding to the measurements of thetwo shutters can be expressed as:g ₁ =γL ₁(T ₁)+g _(offset)g ₂ =γL ₂(T ₂)+g _(offset)

Here, g_(offset) is the pixel offset value (in units of counts), and γis the pixel gain value (in units of counts per radiance unit). Thesolutions of these two equations with respect to the two unknownsg_(offset) and γ can be obtained if the values of g₁ and g₂ and theradiance values L₁ and L₂ are available. These values can, for example,be either measured by a reference instrument or calculated from theknown temperatures T₁ and T₂ together with the known spectral responseof the optical system and FPA. For any subsequent measurement, one canthen invert the equation(s) above in order to estimate the radiancevalue of the object from the detector measurement, and this can be donefor each pixel in each FPA within the system.

As already discussed, and in reference to FIGS. 1 through 4, thefield-reference apertures may be disposed in an object space or imagespace of the optical system, and dimensioned to block a particularportion of the IR radiation received from the object. In variousimplementations, the field-reference aperture, the opening of which canbe substantially similar in shape to the boundary of the filter array(for example, and in reference to a filter array of FIGS. 3B, 5B—e.g.,rectangular). The field-reference aperture can be placed in front of theobjective lens (124, 224, 324, 424) at a distance that is at leastseveral times (in one implementation—at least five times) larger thanthe focal length of the lens such that the field-reference aperture isplaced closer to the object. Placing the field-reference aperture closerto the object can reduce the blurriness of the image. In the embodiment400 of FIG. 4, the field-reference aperture can be placed within thedepth of focus of an image conjugate plane formed by the front objectivelens 424. The field reference, generally, can facilitate, effectuatesand/or enable dynamic compensation in the system by providing aspectrally known and temporally-stable object within every scene toreference and stabilize the output from the different FPAs in the array.

Because each FPA's offset value is generally adjusted from each frame tothe next frame by the hardware, comparing the outputs of one FPA withanother can have an error that is not compensated for by the staticcalibration parameters g_(offset) and γ established, for example, by themovable shutters 160. In order to ensure that FPAs operate inradiometric agreement over time, it is advantageous for a portion ofeach detector array to view a reference source (such as the fieldreference 338 in FIG. 3A, for example) over a plurality of framesobtained over time. If the reference source spectrum is known a priori(such as a blackbody source at a known temperature), one can measure theresponse of each FPA to the reference source in order to estimatechanges to the pixel offset value. However, the temperature of thereference source need not be known. In such implementations, dynamiccalibration of the different detectors can be performed by monitoringthe change in the gain and the offset for the various detectors from thetime the movable shutters used for static calibration are removed. Anexample calculation of the dynamic offset proceeds as follows.

Among the FPA elements in an array of FPAs in an embodiment of theimaging system, one FPA can be selected to be the “reference FPA”. Thefield reference temperature measured by all the other FPAs can beadjusted to agree with the field reference temperature measured by thereference as discussed below. The image obtained by each FPA includes aset of pixels obscured by the field reference 338. Using the previouslyobtained calibration parameters g_(offset) and γ (the pixel offset andgain), the effective blackbody temperature T_(i) of the field referenceas measured by each FPA is estimated using the equation below:T _(i)=mean{(g+Δg _(i) +g _(offset)/γ}=mean{(g−g _(offset))/γ}+ΔT _(i)

Using the equation above, the mean value over all pixels that areobscured by the field reference is obtained. In the above equationΔg_(i) is the difference in offset value of the current frame fromΔg_(offset) obtained during the calibration step. For the reference FPA,Δg_(i) can be simply set to zero. Then, using the temperaturedifferences measured by each FPA, one obtainsT _(i) −T _(ref)=mean{(g+Δg _(i) +g _(offset) /γ}+ΔT _(i)−mean{(g−g_(offset))/γ}=ΔT _(i)

Once ΔT_(i) for each FPA is measured, its value can be subtracted fromeach image in order to force operational agreement between such FPA andthe reference FPA. While the calibration procedure has been discussedabove in reference to calibration of temperature, a procedurally similarmethodology of calibration with respect to radiance value can also beimplemented.

Examples of Methodology of Measurements.

Prior to optical data acquisition using an embodiment of the IR imagingsystem as described herein, one or more, most, or potentially all theFPAs of the system can be calibrated. For example, greater than 50%,60%, 70%, 80% or 90% or the FPAs 336 can be initially calibrated. Asshown in FIG. 3, these FPAs 336 may form separate images of the objectusing light delivered in a corresponding optical channel that mayinclude the combination of the corresponding front objective andre-imaging lenses 324, 128. The calibration procedure can allowformation of individual images in equivalent units (so that, forexample, the reading from the FPA pixels can be re-calculated in unitsof temperature or radiance units, etc.). Moreover, the calibrationprocess can also allow the FPAs (e.g., each of the FPAs) to be spatiallyco-registered with one another so that a given pixel of a particular FPAcan be optically re-mapped through the optical system to the samelocation at the object as the corresponding pixel of another FPA.

To achieve at least some of these goals, a spectral differencing methodmay be employed. The method involves forming a difference image fromvarious combinations of the images from different channels. Inparticular, the images used to form difference images can be registeredby two or more different FPAs in spectrally distinct channels havingdifferent spectral filters with different spectral characteristics.Images from different channels having different spectral characteristicswill provide different spectral information. Comparing (e.g.,subtracting) these images, can therefore yield valuable spectral basedinformation. For example, if the filter element of the array of spectralfilters 130 corresponding to a particular FPA 336 transmits light fromthe object 110 including a cloud of gas, for example, with a certainspectrum that contains the gas absorption peak or a gas emission peakwhile another filter element of the array of spectral filters 130corresponding to another FPA 336 does not transmit such spectrum, thenthe difference between the images formed by the two FPAs at issue willhighlight the presence of gas in the difference image.

A shortcoming of the spectral differencing method is that contributionsof some auxiliary features associated with imaging (not just the targetspecies such as gas itself) can also be highlighted in and contribute tothe difference image. Such contributing effects include, to name just afew, parallax-induced imaging of edges of the object, influence ofmagnification differences between the two or more optical channels, anddifferences in rotational positioning and orientation between the FPAs.While magnification-related errors and FPA-rotation-caused errors can becompensated for by increasing the accuracy of the instrumentconstruction as well as by post-processing of the acquired imaging,parallax is scene-induced and is not so easily correctable. In addition,the spectral differencing method is vulnerable to radiance calibrationerrors. Specifically, if one FPA registers radiance of light from agiven feature of the object as that having a temperature of 40° C., forexample, while the data from another FPA represents the temperature ofthe same object feature as being 39° C., then such feature of the objectwill be enhanced or highlighted in the difference image (formed at leastin part based on the images provided by these two FPAs) due to suchradiance-calibration error.

One solution to some of such problems is to compare (e.g., subtract)images from the same FPA obtained at different instances in time. Forexample, images can be compared to or subtracted from a reference imageobtained at another time. Such reference image, which is subtracted fromother later obtained images, may be referred to as a temporal referenceimage. This solution can be applied to spectral difference images aswell. For example, the image data resulting from spectral differenceimages can be normalized by the data corresponding to a temporalreference image. For instance, the temporal reference images can besubtracted from the spectral difference image to obtain the temporaldifference image. This process is referred to, for the purposes of thisdisclosure, as a temporal differencing algorithm or method and theresultant image from subtracting the temporal reference image fromanother image (such as the spectral difference image) is referred to asthe temporal difference image. In some embodiments where spectraldifferencing is employed, a temporal reference image may be formed, forexample, by creating a spectral difference image from the two or moreimages registered by the two or more FPAs at a single instance in time.This spectral difference image is then used as a temporal referenceimage. The temporal reference image can then be subtracted from otherlater obtained images to provide normalization that can be useful insubtracting out or removing various errors or deleterious effects. Forexample, the result of the algorithm is not affected by a priorknowledge of whether the object or scene contains a target species (suchas gas of interest), because the algorithm can highlight changes in thescene characteristics. Thus, a spectral difference image can becalculated from multiple spectral channels as discussed above based on asnap-shot image acquisition at any later time and can be subtracted fromthe temporal reference image to form a temporal difference image. Thistemporal difference image is thus a normalized difference image. Thedifference between the two images (the temporal difference image) canhighlight the target species (gas) within the normalized differenceimage, since this species was not present in the temporal referenceframe. In various embodiments, more than two FPAs can be used both forregistering the temporal reference image and a later-acquired differenceimage to obtain a better SNR figure of merit. For example, if two FPAsare associated with spectral filters having the same spectralcharacteristic, then the images obtained by the two FPAs can be combinedafter they have been registered to get a better SNR figure.

While the temporal differencing method can be used to reduce oreliminate some of the shortcomings of the spectral differencing, it canintroduce unwanted problems of its own. For example, temporaldifferencing of imaging data is less sensitive to calibration andparallax induced errors than the spectral differencing of imaging data.However, any change in the imaged scene that is not related to thetarget species of interest (such as particular gas, for example) ishighlighted in a temporally-differenced image. Thus such change in theimaged scene may be erroneously perceived as a location of the targetspecies triggering, therefore, an error in detection of target species.For example, if the temperature of the background against which the gasis being detected changes (due to natural cooling down as the dayprogresses, or increases due to a person or animal or another objectpassing through the FOV of the IR imaging system), then such temperaturechange produces a signal difference as compared to the measurement takenearlier in time. Accordingly, the cause of the scenic temperature change(the cooling object, the person walking, etc.) may appear as thedetected target species (such as gas). It follows, therefore, that anattempt to compensate for operational differences among the individualFPAs of a multi-FPA IR imaging system with the use of methods that turnon spectral or temporal differencing can cause additional problemsleading to false detection of target species. Among these problems arescene-motion-induced detection errors and parallax-caused errors thatare not readily correctable and/or compensatable. Accordingly, there isa need to compensate for image data acquisition and processing errorscaused by motion of elements within the scene being imaged. Variousembodiments of data processing algorithms described herein address andfulfill the need to compensate for such motion-induced andparallax-induced image detection errors.

In particular, to reduce or minimize parallax-induced differencesbetween the images produced with two or more predetermined FPAs, anotherdifference image can be used that is formed from the images of at leasttwo different FPAs to estimate parallax effects. Parallax error can bedetermined by comparing the images from two different FPAs where theposition between the FPAs is known. The parallax can be calculated fromthe known relative position difference. Differences between the imagesfrom these two FPAs can be attributed to parallax, especially, if theFPA have the same spectral characteristics, for example have the samespectral filter or both have no spectral filters. Parallax errorcorrection, however, can still be obtained from two FPAs that havedifferent spectral characteristics or spectral filters, especially ifthe different spectral characteristics, e.g., the transmission spectraof the respective filters are known and/or negligible. Use of more thantwo FPAs or FPAs of different locations such as FPAs spaced fartherapart can be useful. For example, when the spectral differencing of theimage data is performed with the use of the difference between theimages collected by the outermost two cameras in the array (such as, forexample, the FPAs corresponding to filters 2 and 3 of the array offilters of FIG. 5A), a difference image referred to as a “differenceimage 2-3” is formed. In this case, the alternative “difference image1-4” is additionally formed from the image data acquired by, forexample, the alternative FPAs corresponding to filters 1 and 4 of FIG.5A. Assuming or ensuring that both of these two alternative FPAs haveapproximately the same spectral sensitivity to the target species, thealternative “difference image 1-4” will highlight pixels correspondingto parallax-induced features in the image. Accordingly, based onpositive determination that the same pixels are highlighted in thespectral “difference image 2-3” used for target species detection, aconclusion can be made that the image features corresponding to thesepixels are likely to be induced by parallax and not the presence oftarget species in the imaged scene. It should be noted that compensationof parallax can also be performed using images created by individualre-imaging lenses, 128 a, when using a single FPA or multiple FPA's asdiscussed above. FPAs spaced apart from each other in differentdirections can also be useful. Greater than 2, for example, 3 or 4, ormore FPAs can be used to establish parallax for parallax correction. Incertain embodiments two central FPAs and one corner FPA are used forparallax correction. These FPA may, in certain embodiments, havesubstantially similar or the same spectral characteristics, for example,have filters having similar or the same transmission spectrum or have nofilter at all.

Another capability of the embodiments described herein is the ability toperform the volumetric estimation of a gas cloud. This can beaccomplished by using (instead of compensating or negating) the parallaxinduced effects described above. In this case, the measured parallaxbetween two or more similar spectral response images (e.g., two or morechannels or FPAs) can be used to estimate a distance between the imagingsystem and the gas cloud or between the imaging system and an object inthe field of view of the system. The parallax induced transverse imageshift, d, between two images is related to the distance, z, between thecloud or object 110 and the imaging system according to the equationz=−sz′/d. Here, s, is the separation between two similar spectralresponse images, and z′ is the distance to the image plane from the backlens. The value for z′ is typically approximately equal to the focallength f of the lens of the imaging system. Once the distance z betweenthe cloud and the imaging system is calculated, the size of the gascloud can be determined based on the magnification, m=f/z, where eachimage pixel on the gas cloud, Δx′, corresponds to a physical size inobject space Δx=Δx′/m. To estimate the volume of the gas cloud, aparticular symmetry in the thickness of the cloud based on the physicalsize of the cloud can be assumed. For example, the cloud image can berotated about a central axis running through the cloud image to create athree dimensional volume estimate of the gas cloud size. It is worthnoting that in the embodiments described herein only a single imagingsystem is required for such volume estimation. Indeed, due to the factthat the information about the angle at which the gas cloud is seen bythe system is decoded in the parallax effect, the image data includesthe information about the imaged scene viewed by the system inassociation with at least two angles.

When the temporal differencing algorithm is used for processing theacquired imaging data, a change in the scene that is not caused by thetarget species can inadvertently be highlighted in the resulting image.In various embodiments, compensation for this error makes use of thetemporal differencing between two FPAs that are substantially equallyspectrally sensitive to the target species. In this case, the temporaldifference image will highlight those pixels the intensity of which havechanged in time (and not in wavelength). Therefore, subtracting the datacorresponding to these pixels on both FPAs, which are substantiallyequally spectrally sensitive to the target species, to form theresulting image, excludes the contribution of the target species to theresulting image. The differentiation between (i) changes in the scenedue to the presence of target species and (ii) changes in the scenecaused by changes in the background not associated with the targetspecies is, therefore, possible. In some embodiments, these two channelshaving the same or substantially similar spectral response so as besubstantially equally spectrally sensitive to the target species maycomprise FPAs that operate using visible light. It should also be notedthat, the data acquired with a visible light FPA (when present as partof the otherwise IR imaging system) can also be used to facilitate suchdifferentiation and compensation of the motion-caused imaging errors.Visible cameras generally have much lower noise figure than IR cameras(at least during daytime). Consequently, the temporal difference imageobtained with the use of image data from the visible light FPA can bequite accurate. The visible FPA can be used to compensate for motion inthe system as well as many potential false-alarms in the scene due tomotion caused by people, vehicles, birds, and steam, for example, aslong as the moving object can be observed in the visible region of thespectra. This has the added benefit of providing an additional level offalse alarm suppression without reducing the sensitivity of the systemsince many targets such as gas clouds cannot be observed in the visiblespectral region. In various implementations, an IR camera can be used tocompensate for motion artifacts.

Another method for detection of the gases is to use a spectral unmixingapproach. A spectral unmixing approach assumes that the spectrummeasured at a detector pixel is composed of a sum of component spectra(e.g., methane and other gases). This approach attempts to estimate therelative weights of these components needed to derive the measurementspectrum. The component spectra are generally taken from a predeterminedspectral library (for example, from data collection that has beenempirically assembled), though sometimes one can use the scene toestimate these as well (often called “endmember determination”). Invarious embodiments, the image obtained by the detector pixel is aradiance spectrum and provides information about the brightness of theobject. To identify the contents of a gas cloud in the scene and/or toestimate the concentration of the various gases in the gas cloud, anabsorption/emission spectrum of the various gases of interest can beobtained by comparing the measured brightness with an estimate of theexpected brightness. The spectral unmixing methodology can also benefitfrom temporal, parallax, and motion compensation techniques.

In various embodiments, a method of identifying the presence of a targetspecies in the object includes obtaining the radiance spectrum (or theabsorption spectrum) from the object in a spectral region indicative ofthe presence of the target species and calculating a correlation (e.g.,a correlation coefficient) by correlating the obtained radiance spectrum(or the absorption spectrum) with a reference spectrum for the targetspecies. The presence or absence of the target species can be determinedbased on an amount of correlation (e.g., a value of correlationcoefficient). For example, the presence of the target species in theobject can be confirmed if the amount of correlation or the value ofcorrelation coefficient is greater than a threshold. In variousimplementations, the radiance spectrum (or the absorption spectrum) canbe obtained by obtaining a spectral difference image between a filteredoptical channel and/or another filtered optical channel/unfilteredoptical channel or any combinations thereof.

For example, an embodiment of the system configured to detect thepresence of methane in a gas cloud comprises optical components suchthat one or more of the plurality of optical channels is configured tocollect IR radiation to provide spectral data corresponding to adiscrete spectral band located in the wavelength range between about 7.9μm and about 8.4 μm corresponding to an absorption peak of methane. Themultispectral data obtained in the one or more optical channels can becorrelated with a predetermined absorption spectrum of methane in thewavelength range between about 7.9 μm and 8.4 μm. In variousimplementations, the predetermined absorption spectrum of methane can besaved in a database or a reference library accessible by the system.Based on an amount of correlation (e.g., a value of correlationcoefficient), the presence or absence of methane in the gas cloud can bedetected.

Examples of Practical Embodiments and Operation

The embodiment 300 of FIG. 3 is configured to employ 12 optical channelsand 12 corresponding microbolometer FPAs 336 to capture a video sequencesubstantially immediately after performing calibration measurements. Thevideo sequence corresponds to images of a standard laboratory scene andthe calibration measurements are performed with the use of a referencesource including two shutters, as discussed above, one at roomtemperature and one 5° C. above room temperature. The use of 12 FPAsallows increased chance of simultaneous detection and estimation of theconcentrations of about 8 or 9 gases present at the scene. In variousembodiments, the number of FPAs 336 can vary, depending on the balancebetween the operational requirements and consideration of cost.

Due to the specifics of operation in the IR range of the spectrum, theuse of the so-called noise-equivalent temperature difference (or NETD)is preferred and is analogous to the SNR commonly used in visiblespectrum instruments. The array of microbolometer FPAs 336 ischaracterized to perform at NETD≤72 mK at an f-number of 1.2. Eachmeasurement was carried out by summing four consecutive frames, and thereduction in the NETD value expected due to such summation would bedescribed by corresponding factor of √4=2. Under ideal measurementconditions, therefore, the FPA NETD should be about 36 mK.

It is worth noting that the use of optically-filtered FPAs in variousembodiments of the system described herein can provide a system withhigher number of pixels. For example, embodiments including a singlelarge format microbolometer FPA array can provide a system with largenumber of pixels. Various embodiments of the systems described hereincan also offer a high optical throughput for a substantially low numberof optical channels. For example, the systems described herein canprovide a high optical throughput for a number of optical channelsbetween 4 and 50. By having a lower number of optical channels (e.g.,between 4 and 50 optical channels), the systems described herein havewider spectral bins which allows the signals acquired within eachspectral bin to have a greater integrated intensity.

An advantage of the embodiments described herein over various scanningbased hyperspectral systems that are configured for target speciesdetection (for example, gas cloud detection) is that, the entirespectrum can be resolved in a snapshot mode (for example, during oneimage frame acquisition by the FPA array). This feature enables theembodiments of the imaging systems described herein to take advantage ofthe compensation algorithms such as the parallax and motion compensationalgorithms mentioned above. Indeed, as the imaging data required toimplement these algorithms are collected simultaneously with thetarget-species related data, the compensation algorithms are carried outwith respect to target-species related data and not with respect to dataacquired at another time interval. This rapid data collection thusimproves the accuracy of the data compensation process. In addition, theframe rate of data acquisition is much higher. For example, embodimentsof the imaging system described herein can operate at video rates fromabout 5 Hz and higher. For example, various embodiments described hereincan operate at frame rates from about 5 Hz to about 60 Hz or 200 Hz.Thus, the user is able to recognize in the images the wisps and swirlstypical of gas mixing without blurring out of these dynamic imagefeatures and other artifacts caused by the change of scene (whetherspatial or spectral) during the lengthy measurements. Incontradistinction, scanning based imaging systems involve image dataacquisition over a period of time exceeding a single-snap-shot time andcan, therefore, blur the target gas features in the image and inevitablyreduce the otherwise achievable sensitivity of the detection. Thisresult is in contrast to embodiments of the imaging system describedherein that are capable of detecting the localized concentrations of gaswithout it being smeared out with the areas of thinner gasconcentrations. In addition, the higher frame rate also enables a muchfaster response rate to a leak of gas (when detecting such leak is thegoal). For example, an alarm can trigger within fractions of a secondrather than several seconds.

To demonstrate the operation and gas detection capability of the imagingsystems described herein, a prototype was constructed in accordance withthe embodiment 300 of FIG. 3A and used to detect a hydrocarbon gas cloudof propylene at a distance of approximately 10 feet. FIG. 7 illustratesvideo frames 1 through 12 representing gas-cloud-detection output 710(seen as a streak of light) in a sequence from t=1 to t=12. The images 1through 12 are selected frames taken from a video-data sequence capturedat a video-rate of 15 frames/sec. The detected propylene gas is shown asa streak of light 710 (highlighted in red) near the center of eachimage. The first image is taken just prior to the gas emerging from thenozzle of a gas-contained, while the last image represents the systemoutput shortly after the nozzle has been turned off.

The same prototype of the system can also demonstrate the dynamiccalibration improvement described above by imaging the scene surroundingthe system (the laboratory) with known temperature differences. Theresult of implementing the dynamic correction procedure is shown inFIGS. 8A, 8B, where the curves labeled “obj” (or “A”) representtemperature estimates of an identified region in the scene. The abscissain each of the plots of FIGS. 8A, 8B indicates the number of a FPA,while the ordinate corresponds to temperature (in degrees C.).Accordingly, it is expected that when all detector elements receiveradiant data that, when interpreted as the object's temperature,indicates that the object's temperature perceived by all detectorelements is the same, any given curve would be a substantially flatline. Data corresponding to each of the multiple “obj” curves are takenfrom a stream of video frames separated from one another by about 0.5seconds (for a total of 50 frames). The recorded “obj” curves shown inFIG. 8A indicate that the detector elements disagree about the object'stemperature, and that difference in object's temperature perceived bydifferent detector elements is as high as about 2.5° C. In addition, allof the temperature estimates are steadily drifting in time, from frameto frame. The curves labeled “ref” (or “C”) correspond to the detectors'estimates of the temperature of the aperture 338 of the embodiment 300of FIG. 3A. The results of detection of radiation carried out after eachdetector pixel has been subjected to the dynamic calibration proceduredescribed above are expressed with the curved labeled “obj corr” (or“B”). Now, the difference in estimated temperature of the object amongthe detector elements is reduced to about 0.5° C. (thereby improving theoriginal reading at least by a factor of 5).

FIG. 8B represents the results of similar measurements corresponding toa different location in the scene (a location which is at a temperatureabout 9° C. above the estimated temperature of the aperture 338 of FIG.3A). As shown, the correction algorithm discussed above is operable andeffective and applicable to objects kept at different temperature.Accordingly, the algorithm is substantially temperature independent.

Dynamic Calibration Elements and References

FIGS. 9A and 9B illustrates schematically different implementations 900and 905 respectively of the imaging system that include a variety oftemperature calibration elements to facilitate dynamic calibration ofthe FPAs. The temperature calibration elements can include mirrors 975a, 975 b (represented as M_(1A), M_(9A), etc.) as well as referencesources 972 a and 972 b. The implementation 900 can be similarlyconfigured as the embodiment 300 and include one or more front objectivelens, a divided aperture, one or more spectral filters, an array ofimaging lenses 928 a and an imaging element 936. In variousimplementations, the imaging element 936 (e.g., camera block) caninclude an array of cameras. In various implementations, the array ofcameras can comprise an optical FPA unit. The optical FPA unit cancomprise a single FPA, an array of FPAs. In various implementations, thearray of cameras can include one or more detector arrays represented asdetector array 1, detector array 5, detector array 9 in FIGS. 9A and 9B.In various embodiments, the FOV of each of the detector arrays 1, 5, 9can be divided into a central region and a peripheral region. Withoutany loss of generality, the central region of the FOV of each of thedetector arrays 1, 5, 9 can include the region where the FOV of all thedetector arrays 1, 5, 9 overlap. In the embodiment illustrated in FIG.9A, some temperature-calibration elements, namely, reference sources 972a and 972 b are placed at a distance from the detector arrays 1, 5, 9,for example, and mirrors to image them onto the detector arrays are thenplaced at the location of the scene reference aperture (e.g., 338 ofFIG. 3A).

In FIG. 9A, the temperature-calibration elements 975 a and 975 bcomprise mirrors that reflect radiation from the reference sources 972 aand 972 b (represented as ref A and ref B). The mirrors 975 a and 975 bcan be disposed away from the central FOV of the detector arrays 1, 5, 9such that the central FOV is not blocked or obscured by the image of thereference source 972 a and 972 b. In various implementations, the FOV ofthe detector array 5 could be greater than the FOV of the detectorarrays 1 and 9. In such implementations, the mirrors 975 a and 975 b canbe disposed away from the central FOV of the detector array 5 at alocation such that the reference source 972 a and 972 b is imaged by thedetector array 5. The mirrors 975 a and 975 b may comprise imagingoptical elements having optical power that image the reference sources972 a and 972 b onto the detector arrays 1 and 9. In this example, thereference sources 972 a and 972 b can be disposed in the same plane asthe re-imaging lenses 928 a, however, the reference sources 972 a and972 b can be disposed in a different plane or in different locations.For example, the reference sources 972 a and 972 b can be disposed in aplane that is conjugate to the plane in which the detector array 1,detector array 5, and detector array 9 are disposed such that a focusedimage of the reference sources 972 a and 972 b is formed by the detectorarrays. In some implementations, the reference sources 972 a and 972 bcan be disposed in a plane that is spaced apart from the conjugate planesuch that a defocused image of the reference sources 972 a and 972 b isformed by the detector arrays. In various implementations, the referencesources 972 a and 972 b need not be disposed in the same plane.

As discussed above, in some embodiments, the reference sources 972 a and972 b are imaged onto the detector array 1 and detector array 9, withoutmuch blur such that the reference sources 972 a and 972 b are focused.In contrast, in other embodiments, the image of the formed of referencesources 972 a and 972 b on the detector array 1, and detector array 9are blurred such that the reference sources 972 a and 972 b aredefocused, and thereby provide some averaging, smoothing, and/or lowpass filtering. The reference sources 972 a and 972 b may comprise asurface of known temperature and may or may not include a heater orcooler attached thereto or in thermal communication therewith. Forexample, the reference source 972 a and 972 b may comprises heaters andcoolers respectively or may comprise a surface with a temperature sensorand a heater and sensor respectively in direct thermal communicationtherewith to control the temperature of the reference surface. Invarious implementations, the reference sources 972 a and 972 b caninclude a temperature controller configured to maintain the referencesources 972 a and 972 b at a known temperature. In some implementations,the reference sources 972 a and 972 b can be associated with one or moresensors that measure the temperature of the reference sources 972 a and972 b and communicate the measured temperature to the temperaturecontroller. In some implementations, the one or more sensors cancommunicate the measured temperature to the data-processing unit. Invarious implementations, the reference sources 972 a and 972 b maycomprise a surface of unknown temperature. For example, the referencesources may comprise a wall of a housing comprising the imaging system.In some implementations, the reference sources 972 a and 972 bcomprising a surface need not be associated with sensors, temperaturecontrollers. However, in other implementations, the reference sources972 a and 972 b comprising a surface can be associated with sensors,temperature controllers.

In FIG. 9B, the temperature-calibration elements comprisetemperature-controlled elements 972 a and 972 b (e.g., a thermallycontrolled emitter, a heating strip, a heater or a cooler) disposed adistance from the detector arrays 1, 5, 9. In various embodiments, thetemperature-controlled elements 972 a and 972 b can be disposed awayfrom the central FOV of the detector arrays 1, 5, 9 such that thecentral FOV is not blocked or obscured by the image of the referencesource 972 a and 972 b. The radiation emitted from the reference sources972 a and 972 b is also imaged by the detector array 936 along with theradiation incident from the object. Depending on the position of thereference sources 972 a and 972 b the image obtained by the detectorarray of the reference sources can be blurred (or defocused) or sharp(or focused). The images 980 a, 980 b, 980 c, 980 d, 980 e and 980 f ofthe temperature-controlled elements 972 a and 972 b can be used as areference to dynamically calibrate the one or more cameras in the arrayof cameras.

In the implementations depicted in FIGS. 9A and 9 b, the detector arrays1, 5 and 9 are configured to view (or image) both the reference sources972 a and 972 b. Accordingly, multiple frames (e.g., every orsubstantially every frame) within a sequence of images contains one ormore regions in the image in which the object image has known thermaland spectral properties. This allows multiple (e.g., most or each)cameras within the array of cameras to be calibrated to agree with other(e.g., most or every other) camera imaging the same reference source(s)or surface(s). For example, detector arrays 1 and 9 can be calibrated toagree with each other. As another example, detector arrays 1, 5 and 9can be calibrated to agree with each other. In various embodiments, thelenses 928 a provide blurred (or defocused) images of the referencesources 972 a, 972 b on the detector arrays 1 and 9 because the locationof the reference sources are not exactly in a conjugate planes of thedetector arrays 1 and 9. Although the lenses 928 a are described asproviding blurred or defocused images, in various embodiments, referencesources or surfaces are imaged on the detectors arrays 1, 5, 9 withoutsuch blur and defocus and instead are focused images. Additionallyoptical elements may be used, such as for example, the mirrors shown inFIG. 9A to provide such focused images.

The temperature of the reference sources 972 b, 972 a can be different.For example, the reference source 972 a can be at a temperature T_(A),and the reference source 972 b can be at a temperature T_(B) lower thanthe temperature T_(A). A heater can be provided under thetemperature-controlled element 972 a to maintain it at a temperatureT_(A), and a cooler can be provided underneath thetemperature-controlled element 972 b to maintain it at a temperatureT_(B). In various implementations, the embodiments illustrated in FIGS.9A and 9B can be configured to image a single reference source 972instead of two references sources 972 a and 972 b maintained atdifferent temperatures. It is understood that the single referencesource need not be thermally controlled. For example, in variousimplementations, a plurality of detectors in the detector array can beconfigured to image a same surface of at least one calibration elementwhose thermal and spectral properties are unknown. In suchimplementations, one of the plurality of detectors can be configured asa reference detector and the temperature of the surface of the at leastone calibration element imaged by the plurality of detectors can beestimated using the radiance spectrum obtained by the referencedetector. The remaining plurality of detectors can be calibrated suchthat their temperature and/or spectral measurements agree with thereference detector. For example, detector arrays 1 and 9 can becalibrated to agree with each other. As another example, detector arrays1, 5 and 9 can be calibrated to agree with each other.

The reference sources 972 a and 972 b can be coated with a material tomake it behave substantially as a blackbody (for which the emissionspectrum is known for any given temperature). If a temperature sensor isused at the location of each reference source, then the temperature canbe tracked at these locations. As a result, the regions in the image ofeach camera (e.g., on the detector arrays 1 and 9) in which the objecthas such known temperature (and, therefore, spectrum) can be defined. Acalibration procedure can thus be used so that most of the cameras (ifnot every camera) so operated agrees, operationally, with most or everyother camera, for objects at the temperatures represented by those twosources. Calibrating infrared cameras using sources at two differenttemperatures is known as a “two-point” calibration, and assumes that themeasured signal at a given pixel is linearly related to the incidentirradiance. Since this calibration can be performed during multiple,more, or even every frame of a sequence, it is referred to as a “dynamiccalibration”.

An example of the dynamic calibration procedure is as follows. If thereis a temperature sensor on the reference sources or reference surface,then the temperature measurements obtained by these temperature sensorscan be used to determine their expected emission spectra. Thesetemperature measurements are labeled as T_(A)[R], T_(B)[R], and T_(C)[R]for the “reference temperatures” of sources/surfaces A, B, and C. Thesetemperature measurements can be used as scalar correction factors toapply to the entire image of a given camera, forcing it to agree withthe reference temperatures. Correcting the temperature estimate of agiven pixel from T to T′ can use formulae analogous to those discussedbelow in reference to FIGS. 10A, 10B, 10C. If no direct temperaturesensor is used, then one of the cameras can be used instead. This cameracan be referred to as the “reference camera”. In this case, the sameformulae as those provided below can be used, but with T_(A)[R] andT_(B)[R] representing the temperatures of the reference sources/surfacesA and B as estimated by the reference camera. By applying the dynamiccalibration correction formulae, all of the other cameras are forced tomatch the temperature estimates of the reference camera.

In the configuration illustrated in FIG. 9B, the reference sources 972 aand 972 b are placed such that the images of the sources on the detectorarrays are blurred. The configuration illustrated in FIG. 9A is similarto the system 400 illustrated in FIG. 4 where the reference sources areplaced at an intermediate image plane (e.g., a conjugate image plane).In this configuration, the array of reference apertures, similar toreference apertures 438 a in FIG. 4, will have an accompanying array ofreference sources or reference surfaces such that the reference sourcesor surfaces (e.g., each reference source or surface) are imaged onto acamera or a detector array such as FPAs 1, 5, 9. With this approach, thereference source or surface images are at a conjugate image plane andthus are not appreciably blurred, so that their images can be made toblock a smaller portion of each camera's field of view.

A “static” calibration (a procedure in which the scene is largelyblocked with a reference source such as the moving shutters 960 in FIGS.9A and 9B, so that imaging of a unknown scene cannot be performed inparallel with calibration) allows a plurality of the cameras (forexample, most or each camera) to accurately estimate the temperature ofa plurality of elements (for example, most or each element in the scene)immediately after the calibration is complete. It cannot, however,prevent the cameras' estimates from drifting away from one anotherduring the process of imaging an unknown scene. The dynamic calibrationcan be used to reduce or prevent this drift, so that all cameras imaginga scene can be forced to agree on the temperature estimate of thereference sources/surfaces, and adjust this correction during everyframe.

FIG. 10A illustrates schematically a related embodiment 1000 of theimaging system, in which one or more mirrors M_(0A), . . . M_(11A) andM_(0B), . . . M_(11B) are placed within the fields of view of one ormore cameras 0, . . . , 11, partially blocking the field of view. Thecameras 0, . . . , 11 are arranged to form an outer ring of camerasincluding cameras 0, 1, 2, 3, 7, 11, 10, 9, 8 and 4 surrounding thecentral cameras 5 and 6. In various implementations, the FOV of thecentral cameras 5 and 6 can be less than or equal to the FOV of theouter ring of cameras 0, 1, 2, 3, 7, 11, 10, 9, 8 and 4. In suchimplementations, the one or more mirrors M_(0A), . . . M_(11A) andM_(OB), . . . M_(11B) can be placed outside the central FOV of thecameras 5 and 6 and is placed in a peripheral FOV of the cameras outerring of cameras 0, 1, 2, 3, 7, 11, 10, 9, 8 and 4 which does not overlapwith the central FOV of the cameras 5 and 6 such that the referencesources A and B are not imaged by the cameras 5 and 6. In variousimplementations, the FOV of the central cameras 5 and 6 can be greaterthan the FOV of the outer ring of cameras 0, 1, 2, 3, 7, 11, 10, 9, 8and 4. In such implementations, the one or more mirrors M_(0A), . . .M_(11A) and M_(OB), . . . M_(11B) can be placed in a peripheral FOV ofthe cameras 5 and 6 which does overlap with the central FOV of the outerring of cameras 0, 1, 2, 3, 7, 11, 10, 9, 8 and 4 such that thereference sources A and B are imaged by the cameras 5 and 6.

This design is an enhancement to the systems 300 and 400 shown in FIGS.3A and 4A. In the system 1000 shown in FIG. 10A, an array of two or moreimaging elements (curved mirrors, for example) is installed at adistance from the FPAs, for example, in the plane of the referenceaperture 160 shown in FIG. 3A. These elements (mirror or imagingelements) are used to image one or more temperature-controlled referencesources A and B onto the detector elements of two or more of thecameras. The primary difference between embodiment 1000 and embodiment300 or 400 is that now a plurality or most or all of the outer ring ofcameras in the array can image both the reference sources A and Binstead of imaging one of the two reference source A and B. Accordingly,most or all of the outer ring of cameras image an identical referencesource or an identical set of reference sources (e.g., both thereference sources A and B) rather than using different reference sourcesfor different cameras or imaging different portions of the referencesources as shown in FIGS. 3A and 4A. Thus, this approach improves therobustness of the calibration, as it eliminates potential failures anderrors due to the having additional thermal sensors estimating eachreference source.

The imaging elements in the system 1000 (shown as mirrors in FIGS. 10Aand 10B) image one or more controlled-temperature reference sources or asurface of a calibration element (shown as A and B in FIGS. 10A and 10B)into the blocked region of the cameras' fields of view. FIG. 10B showsan example in which mirror M_(0A) images reference source/surface A ontocamera 0, and mirror M_(OB) images reference source/surface B ontocamera 0, and likewise for cameras 1, 2, and 3. This way, each of themirrors is used to image a reference source/surface onto a detectorarray of a camera, so that many, most, or every frame within a sequenceof images contains one or more regions in the image in which the objectimage has known thermal and spectral properties. This approach allowsmost of the camera, if not each camera, within the array of cameras tobe calibrated to agree with most or every other camera imaging the samereference source or sources. For example, cameras 0, 1, 2, 3, 7, 11, 10,9, 8 and 4 can be calibrated to agree with each other. As anotherexample, cameras 0, 1, 2 and 3 can be calibrated to agree with eachother. As yet another example, cameras 0, 1, 2, 3, 7, 11, 10, 9, 8, 4, 5and 6 can be calibrated to agree with each other. Accordingly, invarious implementations, two, three, four, five, six, seven, eight,nine, ten, eleven or twelve cameras can be calibrated to agree with eachother. In certain embodiments, however, not all the cameras arecalibrated to agree with each other. For example, one, two, or morecameras may not be calibrated to agree with each other while others maybe calibrated to agree with each other. In various embodiments, thesemirrors may be configured to image the reference sources/surfaces A andB onto different respective pixels a given FPA. Without any loss ofgenerality, FIGS. 10A and 10B represent a top view of the embodimentshown in FIG. 9A.

FIG. 10C illustrates schematically a related embodiment 1050 of theimaging system, in which one or more′ reference sources R_(0A), . . . ,R_(11A) and R_(OB), . . . R_(11B) are disposed around the array ofdetectors 0, . . . , 11. In various implementations, the one or morereference sources R_(0A), . . . R_(11A) and R_(OB), . . . , R_(11B) canbe a single reference source that is images by the detectors 0, . . . ,11. In various implementations, central detector arrays 5 and 6 can havea FOV that is equal to or lesser than the FOV of the outer ring of thedetectors 0, 1, 2, 3, 7, 11, 10, 9, 8 and 4. In such implementations,the reference sources R_(0A), . . . , R_(11A) can be disposed away fromthe central FOV of the detector arrays 5 and 6 such that the radiationfrom the reference sources R_(0A), . . . , R_(11A) is imaged only by theouter ring of detectors 0, 1, 2, 3, 7, 11, 10, 9, 8 and 4. In variousimplementations, central detector arrays 5 and 6 can have a FOV that isgreater than the FOV of the outer ring of the detectors 0, 1, 2, 3, 7,11, 10, 9, 8 and 4. In such implementations, the reference sourcesR_(0A), . . . , R_(11A) can be disposed in the peripheral FOV of thedetector arrays 5 and 6 such that the radiation from the referencesources R_(0A), . . . , R_(11A) is imaged only by the outer ring ofdetectors 0, 1, 2, 3, 7, 11, 10, 9, 8 and 4. The radiation from thereference sources R_(0A), . . . , R_(11A) is therefore imaged by theouter ring of detectors 0, 1, 2, 3, 7, 11, 10, 9, 8 and 4 as well ascentral cameras 5 and 6. Without any loss of generality, FIG. 10Crepresents a top view of the embodiment shown in FIG. 9B.

In various implementations, one can use a heater underneath, adjacentto, or in thermal communication with reference source/surface A to giveit a higher temperature T_(A), and a cooler underneath, adjacent to, orin thermal communication with reference source B to give it a lowertemperature T_(B). In various implementations, the embodimentsillustrated in FIGS. 10A, 10B and 10C can be configured to image asingle reference source A instead of two references sources A and Bmaintained at different temperatures. As discussed above, theembodiments illustrated in FIGS. 10A, 10B and 10C can be configured toimage a same surface of a calibration element. In such implementations,the temperature of the surface of the calibration element need not beknown. Many, most or each reference source/surface can be coated with amaterial to make it behave approximately as a blackbody, for which theemission spectrum is known for any given temperature. If many, most, oreach camera in the array of cameras images both of references A and B,so that there are known regions in the image of these cameras in whichthe object has a known temperature (and therefore spectrum). Then onecan perform a calibration procedure. This procedure can provide thatmany, most or every camera so operated agrees with various, most, orevery other camera, for objects at the temperatures represented by thosetwo sources. For example, two, three, four, five, six, seven, eight,nine, ten, eleven or twelve cameras can be calibrated to agree with eachother. In certain embodiments, however, not all the cameras arecalibrated to agree with each other. For example, one, two, or morecameras may not be calibrated to agree with each other while others maybe calibrated to agree with each other. As discussed above, calibrationof infrared cameras using sources at two different temperatures is knownas a “two-point” calibration, and assumes that the measured signal at agiven pixel is linearly related to the incident irradiance.

The dynamic calibration is used to obtain a corrected temperature T′from the initial temperature T estimated at each pixel in a camera usingthe following formulae:T′[x,y,c]=(T[x,y,c]−T _(A) [R])G[c]+T _(A) [R]where is T_(A)[R] is a dynamic offset correction factor, and,

${{G\lbrack c\rbrack} = \frac{{T_{B}\lbrack R\rbrack} - {T_{A}\lbrack R\rbrack}}{{T_{B}\lbrack c\rbrack} - {T_{A}\lbrack c\rbrack}}},$is a dynamic gain correction factor. The term c discussed above is acamera index that identifies the camera whose data is being corrected.

References throughout this specification to “one embodiment,” “anembodiment,” “a related embodiment,” or similar language mean that aparticular feature, structure, or characteristic described in connectionwith the referred to “embodiment” is included in at least one embodimentof the present invention. Thus, appearances of the phrases “in oneembodiment,” “in an embodiment,” and similar language throughout thisspecification may, but do not necessarily, all refer to the sameembodiment. It is to be understood that no portion of disclosure, takenon its own and in possible connection with a figure, is intended toprovide a complete description of all features of the invention.

In the drawings like numbers are used to represent the same or similarelements wherever possible. The depicted structural elements aregenerally not to scale, and certain components are enlarged relative tothe other components for purposes of emphasis and understanding. It isto be understood that no single drawing is intended to support acomplete description of all features of the invention. In other words, agiven drawing is generally descriptive of only some, and generally notall, features of the invention. A given drawing and an associatedportion of the disclosure containing a description referencing suchdrawing do not, generally, contain all elements of a particular view orall features that can be presented is this view, for purposes ofsimplifying the given drawing and discussion, and to direct thediscussion to particular elements that are featured in this drawing. Askilled artisan will recognize that the invention may possibly bepracticed without one or more of the specific features, elements,components, structures, details, or characteristics, or with the use ofother methods, components, materials, and so forth. Therefore, althougha particular detail of an embodiment of the invention may not benecessarily shown in each and every drawing describing such embodiment,the presence of this detail in the drawing may be implied unless thecontext of the description requires otherwise. In other instances, wellknown structures, details, materials, or operations may be not shown ina given drawing or described in detail to avoid obscuring aspects of anembodiment of the invention that are being discussed. Furthermore, thedescribed single features, structures, or characteristics of theinvention may be combined in any suitable manner in one or more furtherembodiments.

The features recited in claims appended to this disclosure are intendedto be assessed in light of the disclosure as a whole, including featuresdisclosed in prior art to which reference is made.

At least some elements of a device of the invention can becontrolled—and at least some steps of a method of the invention can beeffectuated, in operation—with a programmable processor governed byinstructions stored in a memory. The memory may be random access memory(RAM), read-only memory (ROM), flash memory or any other memory, orcombination thereof, suitable for storing control software or otherinstructions and data. Those skilled in the art should also readilyappreciate that instructions or programs defining the functions of thepresent invention may be delivered to a processor in many forms,including, but not limited to, information permanently stored onnon-writable storage media (e.g. read-only memory devices within acomputer, such as ROM, or devices readable by a computer I/O attachment,such as CD-ROM or DVD disks), information alterably stored on writablestorage media (e.g. floppy disks, removable flash memory and harddrives) or information conveyed to a computer through communicationmedia, including wired or wireless computer networks. In addition, whilethe invention may be embodied in software, the functions necessary toimplement the invention may optionally or alternatively be embodied inpart or in whole using firmware and/or hardware components, such ascombinatorial logic, Application Specific Integrated Circuits (ASICs),Field-Programmable Gate Arrays (FPGAs) or other hardware or somecombination of hardware, software and/or firmware components.

While examples of embodiments of the system and method of the inventionhave been discussed in reference to the gas-cloud detection, monitoring,and quantification (including but not limited to greenhouse gases suchas Carbon Dioxide, Carbon Monoxide, Nitrogen Oxide as well ashydrocarbon gases such as Methane, Ethane, Propane, n-Butane,iso-Butane, n-Pentane, iso-Pentane, neo-Pentane, Hydrogen Sulfide,Sulfur Hexafluoride, Ammonia, Benzene, p- and m-Xylene, Vinyl chloride,Toluene, Propylene oxide, Propylene, Methanol, Hydrazine, Ethanol,1,2-dichloroethane, 1,1-dichloroethane, Dichlorobenzene, Chlorobenzene,to name just a few), embodiments of the invention can be readily adaptedfor other chemical detection applications. For example, detection ofliquid and solid chemical spills, biological weapons, tracking targetsbased on their chemical composition, identification of satellites andspace debris, ophthalmological imaging, microscopy and cellular imaging,endoscopy, mold detection, fire and flame detection, and pesticidedetection are within the scope of the invention.

As used herein, a phrase referring to “at least one of” a list of itemsrefers to any combination of those items, including single members. Asan example, “at least one of: a, b, or c” is intended to cover: a, b, c,a-b, a-c, b-c, and a-b-c.

If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. The steps of a method or algorithm disclosedherein may be implemented in a processor-executable software modulewhich may reside on a computer-readable medium. Computer-readable mediaincludes both computer storage media and communication media includingany medium that can be enabled to transfer a computer program from oneplace to another. A storage media may be any available media that may beaccessed by a computer. By way of example, and not limitation, suchcomputer-readable media may include RAM, ROM, EEPROM, CD-ROM or otheroptical disk storage, magnetic disk storage or other magnetic storagedevices, or any other medium that may be used to store desired programcode in the form of instructions or data structures and that may beaccessed by a computer. Also, any connection can be properly termed acomputer-readable medium. Disk and disc, as used herein, includescompact disc (CD), laser disc, optical disc, digital versatile disc(DVD), floppy disk, and blu-ray disc where disks usually reproduce datamagnetically, while discs reproduce data optically with lasers.Combinations of the above also may be included within the scope ofcomputer-readable media. Additionally, the operations of a method oralgorithm may reside as one or any combination or set of codes andinstructions on a machine readable medium and computer-readable medium,which may be incorporated into a computer program product.

Various modifications to the implementations described in thisdisclosure may be readily apparent to those skilled in the art, and thegeneric principles defined herein may be applied to otherimplementations without departing from the spirit or scope of thisdisclosure. Thus, the claims are not intended to be limited to theimplementations shown herein, but are to be accorded the widest scopeconsistent with this disclosure, the principles and the novel featuresdisclosed herein.

Certain features that are described in this specification in the contextof separate implementations also can be implemented in combination in asingle implementation. Conversely, various features that are describedin the context of a single implementation also can be implemented inmultiple implementations separately or in any suitable subcombination.Moreover, although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

What is claimed is:
 1. An infrared (IR) imaging system, the imagingsystem comprising: an optical system including an optical focal planearray (FPA) unit, the optical system comprising components associatedwith at least two optical channels spatially and spectrally differentfrom one another, each of the at least two optical channels positionedto transfer IR radiation from an object incident on the optical systemtowards the optical FPA unit, the optical FPA unit comprising at leasttwo detectors disposed at a distance from two corresponding focusinglenses; and a data-processing unit configured to obtain a spectraldifference image by comparing images obtained by the at least twodetectors, said infrared imaging system configured to identify a targetspecies based on said spectral difference image.
 2. The imaging systemof claim 1, wherein the imaging system is further configured to identifytarget species based on a comparison of a first spectral differenceimage and a second spectral difference image, said second spectraldifference image acquired by the at least two detectors at a later timefrom the first spectral difference image.
 3. The imaging system of claim1, wherein the FPA unit does not require cooling during normaloperation.
 4. The imaging system of claim 1, wherein the IR radiationcomprises wavelengths between about 1 micron and about 20 microns. 5.The imaging system of claim 1, wherein the object comprises a gas cloud.6. The imaging system of claim 1, wherein the components associated withthe at least two optical channels comprise at least two spectralfilters, each filter positioned in a respective one of said at least twooptical channels.
 7. The imaging system of claim 6, wherein the at leasttwo spectral filters comprise at least one long pass (LP) filter.
 8. Theimaging system of claim 6, wherein the at least two spectral filterscomprise at least one short pass (SP) filter.
 9. The imaging system ofclaim 1, further comprising a movable temperature reference at a knowntemperature configured to be inserted into an optical path from theobject to the FPA unit along at least one of the two optical channels toblock IR radiation and configured to be removed from the optical path.10. The system of claim 9, wherein the data-processing unit isconfigured to calibrate the FPA unit based on the known temperature. 11.The imaging system of claim 9, wherein the movable temperature referencecomprises a first shutter at a first temperature and a second shutter ata second temperature.
 12. The imaging system of claim 9, wherein themovable temperature reference comprises a system having a first portionat a first temperature and a second portion at a second temperature. 13.The imaging system of claim 1, wherein at least one of the two detectorscomprises a microbolometer.
 14. The imaging system of claim 1, whereinthe data-processing unit is configured to obtain the spectral differenceimage by subtracting images obtained by the at least two detectors, saidimages obtained by the at least two detectors having different spectralcharacteristics.
 15. The imaging system of claim 14, wherein thedata-processing unit is configured to compare the obtained spectraldifference image with a reference image.
 16. The imaging system of claim15, wherein the reference image is obtained at a time interval prior toa time interval when the spectral difference image is obtained.